The Twilight V2.2 rules include rules for basic "reservists" (characters can serve a term in the military, then use military skills for subsequent terms' "Secondary Skills").
But there are many other forms of part-time service available in various parts of the world.
"Reservists" - veterans of one or more terms of regular service who go on to serve part-time. The key distinction is they serve a full (or nearly full) term in the regular military before going into reserve service.
"National Guard" - The term is an America-centric term, but includes troops who:
Undergo basic and advanced training similar to regular troops before joining a reserve formation (as in the American National Guard), as well as
troops with a shorter period of active service who go on to serve part time (as with many Finnish, Russian and South Korean troops)
Territorials - Similar to the above, but with relatively shorter active service and less-frequent reserve training.
Home Guard - basically a militia, with relatively minimal training and service requirements.
This section explains them.
Exactly as described in the "Twilight" V2.2 rules.
These soldiers serve one or more terms in the regular military, then devote each subsequent term's secondary activity to the reserve, getting a "subsequent term" skill every term. This is as in Twilight 2000 Version 2.2.
See individual nations for availability of this Reserve career option. Examples of this are the US Army, Navy, Air Force, Marine and Coast Guard Reserve, as well as most of the Israeli Army.
This takes less time. Troops may maintain a civilian career, but spend TWO CONSECUTIVE TERMS secondary activities (The Guard usually involves at least six years' commitment) gaining Basic Training and First Term skills in the Army or Air Force career of their choice. They roll for initiative as reservists. They may then choose to continue serving as active reservists, become inactive (both as explained in the Version 2.2 rules), or leave the service. In wartime, they will serve in their military career.
This path is only available to militaries as depicted in the "Nationalities" section.
In addition, it is the default form of service for most male characters in:
Switzerland
South Korea
Finland
Singapore
...among others, where characters undergo an extensive period of training and active service (but not a full term) before going into the reserve.
Troops may maintain a civilian career, but spend one term's worth of secondary activities) the following from the branch branch of their choice:
Basic training skills
One First Term skill of their choice a level 1, and the rest at Level 0.
They roll for initiative as reservists. They may then choose to continue serving as active reservists, become inactive (both as explained in the Version 2.2 rules), or leave the service. In wartime, they will serve in their military career.
This is also the form of national service used in some countries with lower basic training requirements.
This path is only available to militaries as depicted in the "Nationalities" section.
Home Guard troops spend one secondary activity period learning the following skills:
Small Arms: 1
Melee Combat: 1
They then may serve as Inactive Reservists, rolling for initiative as civilians. In wartime, they generally serve as Internal Security troops (Leg Infantry), gaining wartime skills ONLY from the Leg Infantry Subsequent Term skills charts.
This path is available only in some nations, and it varies by era. Consult the nationalities section for your campaign's era.