Here is the BOM rule from the AMA Control Line Precision Aerobatics rule book.
The BOM rule does not apply to the beginner class and no appearance point will be awarded. (Event 323)
For the Intermediate, Advanced and Expert classes, the contestant does not need to be the builder of the model but no appearance points will be given if the contestant is not the builder of the model. (Events 324, 325 and 326)
Almost all contests in the US are flown to the skill class rules. Some of these contests opt to not have appearance points. Generally, the only contest in the US where the BOM rule applies is Event 322, Precision Aerobatics, at the Nats for the Junior, Senior and Open age groups.
From the AMA rule book:
"2.1. Builder of Model.
The CD shall make every reasonable effort to ascertain that each pilot has
completely “constructed‟ the model(s) the pilot uses in competition, with
“constructed” to be interpreted as the action required to complete a model
starting with no more prefabrication than acquiring 1 of the 2 main
structures preassembled prior to merging and finishing. The 2 main
structures are considered to be: 1) the wing(s) and 2) the fuselage(s).
Flaps, rudders, elevators & horizontal stabilizers are not considered main
structures, therefore there are no limitations on level of their
prefabrication, and only the finish portion of this rule applies to them. In
unconventional stunt designs, such as a multi-engine wing with engine
nacelles, or a flying wing, they are to be considered as multiple merged
structures, so no level of prefabrication is allowed, and on multi wing
planes, the wing total counts as one structure, but the pilot must be the one
who joins and aligns the multiple wings together. In the case of take-apart
models, the take-apart hardware must be installed by the pilot. The pilot
must be the person who applies the finish to the plane, to “finish” meaning
the pilot fills the surfaces and applies the covering and finish to the
completed model where covering and finish is applied. Whereas on the
surface of the main structures, molded structural surface underlayment’s
including but not limited to molded fiberglass, or carbon fiber that are
filled or colored as a result of the manufacturing process that may show as
part of the final finish may be used, as long as this surface underlayment is
applied, filled and colored by the pilot. Control systems such as but not
limited to the bellcrank, control horns, pushrods, etc. may be purchased,
but must be installed by the pilot. Other accessories and hardware may be
purchased or otherwise obtained for their function such as, but not limited
to: engines, tanks, wheels, canopies, airframe take-apart hardware, and
have no bearing in the way “main structures” are counted."
Keith