So this is using a variation of The Cloak's Black Bag RWTS (aka BBRWTS;
I think I have the source code for this, as we used it for various
things.). Yes, $200 is the entry. The part you're likely most
interested in though starts at $2AE. This loads the Track, Sector,
buffer_hi and sector count values from a small list (in that order) and
stores them in the zero page prior to calling the RWTS (whose entry is
at $274; the SEEK_ABS is at $3A6).
There are seven sets in the list. The table is a little different, in
that all the tracks are listed first, then the sectors, then buffer_hi,
and then the counts (rather than in groups of four).
Track $20, sector $0C, buffer $BF00, count #$60
Track $1A, sector $0C, buffer $A500, count #$26
Track $18, sector $06, buffer $A200, count #$03
Track $16, sector $0E, buffer $B400, count #$35
Track $22, sector $0F, buffer $1900, count #$0A
Track $22, sector $05, buffer $1F00, count #$19
Track $18, sector $03, buffer $B400, count #$15
Everything is read in descending order from the starting
track/sector/buffer.
$200 is called with the X-reg loaded with which of the seven to load
(one-based; it's decremented before use as zero-based with the table).
$263 is the prep code for checking which from the list to load.