From the 30,000’ view, tour lengths, “reflect the general desirability of each location and force protection and anti-terrorism considerations,” “full range of facilities both on and off the military installation,” and job (e.g. Defense Attaché Service and Security Cooperation Office, etc). Short tours are typically, “Remote and arduous locations with limited family support facilities or with economic conditions not supporting a quality of life reasonably comparable to U.S. standards,” or, “Locations lacking adequate family support facilities or with potential threat to the safety or security of families.” https://www.esd.whs.mil/Portals/54/Documents/DD/issuances/dodi/131518p.pdf
You can also see the specific factors they consider at the end of that DoD Instruction (population, geography, climate, housing, medical, groceries, banking, etc).
As discovered by Ram, short tour credit is based on the above conditions, not on whether your family is with you or not. Some locations will still allow families. Any location with an unaccompanied tour of 15 months or less on the following list is considered a short tour location: https://www.defensetravel.dod.mil/Docs/perdiem/browse/Travel_Regulations/Appendices/appQ.pdf
Everything else and all AF specific stuff is in the posts SurelySerious linked you to.