1FB is the term derived from the old Bell System USOC codes and is a basic business line. In the old days a 1FL was a line that appeared on a key system (and thus was "lit") and could have options for hunting, which was not originally available on a 1FB. The "B" was/is for business line service (a 1FR was Residential service) and the "F" was for flat rated lines (as opposed to measured line service, which was a 1MB). Some Telco's still use these designations, although the old GTE call the basic business line a "B1" line.
As stated, there were some feature and pricing differences between the various types. But all were considered POTS as opposed to engineered services such as trunk lines used for PBX systems.
This naming goes back to well before the introduction of ISDN and similar services. However, will 1FB is always considered as a POTS line, POTS is not exclusively a 1FB.
