In ASA, DP = Designated Player; DEFO = Defense Only.
On this web site,
Gulf Coast Blue Handouts are two papers on the ASA DP/DEFO rule, which is basically the same as the NFHS Flex rule and the NCAA DP/TP rule. One of these articles was written by SteveM and the other by me.
Read them & check back here if you have more questions.
Trust me ... the rule is actually pretty simple; it just seems complex at first.