Just to clarify..these are the options in Options > Game
[enabled - manual save] This option allows you to save a replay ONCE between the beginning of a race (or the moment you joined the server, mid-race) and the beginning of the NEXT race. To save it, you must press 2 and provide a name for the replay. Once you've done so, you cannot save another replay until the next race starts, or you leave and re-join the server. If you're ejected from a server before having the chance to save a replay, you can find that replay in the [LFS]\Data\mpr directory, saved as temp.mpr. You can rename this file to prevent it being lost. If you join a new server without doing so, the temp.mpr file content will be overwritten.
[not enabled - not stored] This option means that you cannot save replays by pressing 2, nor by searching in the [LFS]\Data\mpr directory. If this option is selected and you want a replay of a race you drove, you will need to ask another racer to send it to you either by email, or post it on the forum for you, or upload it to webspace somewhere and give you a link to go get it.
[auto save - not recommended] This option records all races automatically. It records (and stores) from when you join a server until the beginning of the next race, and then each individual subsequential race after that until you leave the server. If you leave a server before the end of a race, that race is recorded up until that point. The reason this option is not recommended is because it consumes disk space in the background. The files are automatically named in the following format TrackName_NumberOfLaps_NumberOfRacers_NumberOfFinishers[_x]. The _x is an incrementing number added to the end of the file to prevent files being overwritten track_laps_racer_finisher_1, track_laps_racer_finisher_2 etc.
For Single Player recordings, the above applies by and large, but substitute "mpr" with "spr".