A replay or in short a rec is a file where all the moves and timings of a single ride are saved. In Belma/EOL the replay name can be up to 15 characters long (older versions only supported 8 characters).
There is a possibility in Elma to merge two replays together as if they were played simultaneously in the two-player mode. Merging two replays played on different levels is possible, but both replays are shown on the first replay's level. It also might result in an internal error if the other replay goes beyond certain boundaries of the first replay's level.
If you get an internal error when you are saving a rec, you probably don't have a Rec folder or it's write protected.