Label lines begin with the ampersand, ‘&’:
& SGR: SELECT GRAPHIC RENDITION
Label lines always describe a control function, and are always preceded by the escape sequence that invokes that function (unless the -E option was given, suppressing output for escape-sequence lines).
The format of a label line's content is always acronym: name; both of these are defined for each control function by Ecma-48 / ISO/IEC 6429. If the -x option was specified to teseq, then some labels may also name private functions defined by DEC for the VT100 and similar terminals.
Label lines currently are never wrapped; however, future versions of Teseq may wrap label lines. At that time, continuation label lines will probably consist of an extra space after the ampersand (similar to how escape-sequence lines are wrapped). Lines will never be split in the middle of a word.
Future versions of Teseq may use label lines to describe things besides escape sequences; for instance, control characters or control strings, or other strings that may be interpreted specially by some devices or applications.