This corresponds to $7F0C in the ZX Spectrum version.

Used by the routines at $31EA and $36E3. Each byte of the SRB corresponds to a segment of 8 character cells on the screen, the leftmost of which will be at x=0, x=8, x=16, x=24 or x=32. Each bit set in an SRB byte corresponds to a cell in the segment that needs refreshing. Bit 7 corresponds to the leftmost cell in the segment, bit 0 to the rightmost cell.