"Alts" is a colloquial name for "Alternate Universe" "or alternate world"- "Alts" are usually reached via Rings on the Alt Platforms

"Alts" can be alternate histories of the prime world - this is the default. But Alt Rings also can lead to pocket dimensions or specific worlds of interest to the last user.

How the Fringepath System knows the Alts from the Prime Worlds remains mysterious.