This is under Section IX of the Rulebook
Part 6 - General Show Rules
D. Cross-entering:
2. No cross-entering will be allowed between Classic
and Foundation classes with the exception of youth
halter, color, group halter, and Classic performance
classes.
Part 9 – Foundation Classes
... No cross entering into Open Classic classes with the exception of Classic
performance, youth, color, and group halter classes.
I thought this also extended to Amateur classes if a Foundation Amateur class is not offered, but I don't see that listed. The Steward could tell you.