Final answer:
There are no general provisions in military regulations that allow soldiers to wear their shirts un-tucked; dress codes usually require a neat and professional appearance. Soldiers would typically need to tuck in their shirts unless specific, non-standard circumstances permit otherwise.
Step-by-step explanation:
The question regarding which soldiers are allowed to wear their shirts un-tucked is rooted in specific military regulations and practices. Historically, soldiers on leave were required to wear uniforms so that military police (MPs) could easily identify them. This was done to regulate their behavior while off base.
However, there has been a history of servicemen trying to evade these restrictions, for example by using locker clubs to secure civilian clothes while on leave. As for wearing uniforms with shirts un-tucked, this is generally not allowed as military dress codes are quite strict and require a neat and professional appearance.
There are no specific provisions that allow soldiers to wear their shirts un-tucked, and doing so would likely be considered a violation of dress code except potentially under unusual or specific circumstances not outlined in standard regulations.