Barre

"In standard musical notation the barre technique is commonly indicated by the use of one of the two letters: "B" and "C" followed by a positional indication which is given by an Arabic number or Roman numeral. Examples: BIII, CVII, B2, C7.
The two abbreviations "B", "C", represent the terms barre or bar, cejillo or capotasto, the later being Spanish and Italian terms for capo. The choice of letter adopted is an editorial decision reflecting the style adopted. The use of Roman numerals is more prevalent than Arabic numbers to avoid confusion with other fingering indications and common chord symbols (but not figured harmony).
Notation of Roman numerals without including letters "B" or "C" is an indication of fingerboard position only."
(Source: excerpt of this article: https://en.wikipedia.org/wiki/Barre_chord)