Locator (bell) code data
Locator code data is used by GPO for typesetting government documents, notably the US Code. The system was created in the early 1980's and is still in use today. It is basically a highly modal system that uses escape sequences to encode typesetting commands, similar to procedural markup coding used in older word-processing software. The documentation here is somewhat aged material obtained from the House LRC by the LII in 2000 or so.