SPRINTF is a way to format strings. It can be as simple a short string or quite complex specifying field sizes and pad characters for a number. This routine is written in C and is part of the firmware. As of 8.11 firmware BASIC programs can access this powerful C subroutine.
Rather than duplicate the documentation of SPRINTF, there is a great deal of information for printf in documentation for C such as
SPRINTF in the firmware supports c (BYTE), s (STRING), d (decimal INTEGER, x or X for (hex INTEGER), e, E, f, g, or G (for SINGLE).
The G format differs a little from the G format of C. It copies 0.0 or the range 0.1000000 through 9999999. and outside that it uses D.DDDDDDE+DD and the negative values as well.
Pad characters of 0 or space can be added to fill out LENGTH or WIDTH and PRECISION. PRECISION is limited to 0 to 8 characters.
Special characters can be added using %c and filling that value with something like 13 for carriage return, 9 for TAB or 7 to wring the BELL