Is it acceptable to wear a hat indoors?
Historically, men's hat etiquette has designated that all hats should be removed upon entering indoors, which includes houses of worship (unless customary otherwise like at Jewish synagogues), public buildings, and private homes, especially at mealtimes.
Marines do not wear berets. Marines wear boots only with the utility uniform, not other uniforms. Marines do not salute unless they are wearing a hat (known as a "cover"). Marines do not wear covers indoors, unless they are "under arms", i.e. carrying a weapon or wearing a duty belt.
According to the etiquette experts over at the Emily Post Institute, the act of removing your hat indoors is a longtime sign of respect. In fact, it probably began with medieval knights.
Historically, men's hat etiquette has designated that all hats should be removed upon entering indoors, which includes houses of worship (unless customary otherwise like at Jewish synagogues), public buildings, and private homes, especially at mealtimes.
These informal gestures demonstrate your character and respect for Army Values. Uncovering—removing a beret, hat, or headgear—isn't just good manners; it's a sign of respect to others. You should remove your headgear indoors, unless you are under arms.
Officers and Enlisted are identified by their uniforms. Their hats are an important part this uniform. Sailors must wear a hat anytime they are outside, and they take them off when they are inside. One way to tell an officer from an enlisted sailor is to look at the hat he or she is wearing.
Ball caps with titles are not authorized for wear with civilian attire. Visor ornamentation, standard gold color, is authorized for Officers.
On or off? A gentleman will remove his hat indoors, a lady may keep hers on anywhere and anytime (even in church, during a wedding, baptism, or funeral).
Ordinary civilians also salute informally to greet or acknowledge the presence of another person; such as a tip of the hat or a hand wave to a friend or neighbour.
Civilians, those not wearing headgear, and those indoors do not salute superior officers. There is one exception to the “no salutes indoors” rule–when a military member is ordered to report to an officer, a salute is rendered with or without headgear.
Can you wear the black parka in civilian clothes?
When the Rank insignia is removed from the Parka, Personnel may wear the coat with civilian clothing.
WEARING UNIFORM ARTICLES WITH CIVILIAN CLOTHES. Military personnel may wear military uniform articles with civilian clothing when the item cannot be identified as a navy uniform component.

Do not wear a military uniform if you are a civilian. If you have never served in the armed forces, you are prohibited by the United States government from wearing the uniform of the Air Force, Army, Navy, or Marines.
Military traditions
For example, when dressed in a formal uniform hat, a member must wear it outside, remove and carry it inside and keep it off in any military aircraft. In addition, service members are not allowed to salute while their hats are off.
Drill sergeants have the responsibility to preserve and maintain the campaign hat. Drill sergeants will not wear the campaign hat inside facilities of the installation that are outside of their battalion/brigade footprints.
Non-inclusive examples of appropriate civilian attire: (1) shirt or blouse with a collar, (2) trousers/slacks with a belt (if applicable), and (3) socks and shoes.
TLDR – In the United States, it is legal for civilians to wear military uniforms. However, it is against the law to impersonate a member of the military for personal gains, such as wearing a uniform to commit fraud.
"Black Hat," a term used to identify a Soldier as an Airborne School instructor, refers to the black baseball caps worn by instructors. "This is one of the most prestigious awards for a Black Hat," said SGM Michael Green, the senior NCO for 1st Bn., 507th PIR.
You may even be wondering, “Is it okay to wear a beret inside?" The answer is a resounding yes! Especially since its introduction to military uniforms, the beret has been perceived as a dignified style that is suitable for wear in a variety of situations. Express Yourself with a Beret!
Drill instructors are referred to as "sir" or "ma'am" by recruits within the USAF, USMC, and USCG (for the first few weeks of basic training, until recruits are instructed to refer to their company commanders by their proper rank). Within the USN, recruits must refer to their RDCs by their proper ranks.
What does it mean if a drill sergeant has a blue ring on his hat?
Infantry Soldiers wear an infantry blue disc under the seal. Drill sergeants first wore this hat in 1964 as a way of distinguishing themselves from those whom they were charged with transforming into Soldiers. It has been their proud symbol ever since.
For men, yes. For women, no. Men have always removed it as a sign of respect to their host. If deemed to be in an indoor 'public' place such as lobbies, corridors or elevators hats would be kept on for the sake of personal hygiene.
Because members are required to maintain professionalism in uniform, personal displays of affection are typically frowned upon except in certain situations. For instance, moderate kissing and hugging is acceptable when there's a homecoming or deployment.
Do not expect or offer public displays of affection whenever a service member is in uniform. However, brief kisses and hugs are acceptable during deployments and homecomings. Eating, drinking, using a cellphone and smoking while walking is generally banned in uniform.
When in civilian clothes (indoors or outdoors), stand at attention, face the flag, and recite the Pledge of Allegiance while holding your right hand over your heart. (Men should remove their headdress with their right hand and place their right hand, while holding the headdress, over their heart.)
Hats are to be removed when inside, except for places that are akin to public streets, e.g., lobbies, corridors, and elevators in public buildings. In public buildings, the elevator is considered a public area, and therefore an area where a gentleman may leave his hat on.
civilian clothes in American English
1. everyday or ordinary clothing, as distinguished from a military uniform. informal. ordinary clothing as distinguished from a uniform, clerical garb, or work clothes.