I have uploaded here a PDF copy of the USFM User Reference Guide. usfmReference2_4
Version 2.4 of the standard is widely used, and practically every Bible translation tool and online resource supports it. Some software tools (e.g. translationCore, Paratext) make use of the latest USFM 3.0 standard, which is documented at https://ubsicap.github.io/usfm/