When active duty military service member gets official orders to relocate to a different military base, that is considered a "Permanent Change of Station", (PCS). Unless pre-empted by another PCS order or completion of active duty service, all family members living with her or him must adhere to the relocation order.
"Permanent Change of Assignment", (PCA) is the reassignment of an active duty military member to a new unit within the same military post (typically stateside). For example my Father was in The United States Army for 22 years, 10 of them oversees. In 1952 he received his "PCA" papers for Korea where he served in The Korean War.  Ten years later my parents resided in the United States in Portland, Oregon only to receive (PCS) papers. (PCS) stands for "Permanent Change of Station." In this case my Father, now Retired Lt Col. was served with PCS papers in 1963 for The Vietnam War where he had a 12 month tour. 

In listening to my Mother's faint memories she always emphasized the importance of "being prepared to leave and adaptable to change." "I never let any grass grow under my feet, she said...you have 2-3 weeks to get it all together and if you don't have it, it ain't going!" The most difficult part she said, "isn't the move but, wondering whether her husband was going to make it back home safely..."
If you are a military service member or spouse that just received PCS/PCA papers please see our "Buddy System"which is designed to make your transition as smooth as possible.