It means the engine is new and not yet used enough to run its best.

Here is the link for 3mm.

http://bcmaengines.com/BCMA3mm53.htm

Look at all of the engines available before you buy. You decide which is best for you price wise and performance wise. If you can afford the best then buy the best. If you want a good engine at a good price then shop a bit. Your choice, your decision, your engine.