ORIGINAL: Silent-AV8R

Differences:

Pattern flies dedicated designs specific to the task of flying precision aerobatics
IMAC flies scale (ish). +/- 10% deviation from scale allowed.

Pattern planes are only allowed to be up to 2-meters in span and 11 pounds total weight
IMAC planes can be up to 55 pounds (more if a waiver has been obtained by the owner)

Pattern planes are typically powered by either 1.40 cu. in. 4-cycle or 10S electric motors
IMAC is usually 100cc to 170cc gasoline powered

Pattern flies a longer sequence (17 to 23 figures). Figures are basically whatever sounds good to fly. Like a triangle loop for instance.
IMAC flies 10 figure sequences based on the current FAI/Aresti catalog of aerobatic figures.

Pattern flies a single sequence per flight/round
IMAC typically flies 2 sequences per flight/round

Both end up with 6 scored sequences per contest (typically)
Both normalize scores for the round and drop the lowest 2 rounds.

Pattern does not fly an Unknown (Except in FAI at the NATS and World)
IMAC flies an Unknown in Sportsman and above.

Pattern sequences change every two to three years.
IMAC sequences change every year.

Pattern scores based on 1 point per 15 degrees of error
IMAC scores based on 1/2 point per 5 degrees of error

Pattern sequence are flown in a defined aerobatic box with a centering requirement.
IMAC has no box or centering requirement

IMAC is the AMA Special Interest Group (SIG) for scale aerobatics
NSRCA (National Society of Radio Controlled Aerobatics) is the AMA SIG for aerobatics (except scale).

Pattern is the group from which the US F3A Team is selected to represent the US
IMAC presently has no International/World Level event among the various countries of the world

Both are a lot of fun.