A horse is either CLEAN or DIRTY. (But never both.)
In Playlist file mode this can be written into the UDM definition using preset filter codes CLEAN and DIRTY.
In SQL Mode there is a table field named CANTRUNXFORMATTRIB.
In SQL Mode, clean horses are indicated in a UDM definition using a line of sql like the following:
AND CANTRUNXFORMATTRIB = 0
Dirty horses in SQL Mode are indicated in a UDM Definition using the following line of sql:
AND CANTRUNXFORMATTRIB > 0
PctDirty is simply the percentage of DIRTY horses in a given race. For example, if 2 starters in a field of 8 are DIRTY, then PctDirty is 25 (or 2 divided by 8 = 25 percent.)