Is that what 6+6 mean? 6pm to 6am?
Not quite. The 6+6 category was originally introduced for riders aged 16 and 17 (because you can't do an MTBA solo 24hr until you are 18 in Australia). It allowed them to get the feel of endurance racing.
It was then expanded to allow older riders as well.
The plan is that the 6+6 categories start with the main field at 1200 Saturday. They then stop on their first lap which finishes after 6pm Saturday, then have the night off. Some go home, some stay at the race to converse with other mountain bikers and provide constructive criticism to the riders still on course
They start again on Sunday morning 12 hours later in a staggered start format. So, if you finished your last lap on Saturday at 18:10:45 then the starter would release you onto the course at 06:10:45 on Sunday morning. You then finish with the main field around 1200 Sunday.
If you wanted to use the race for some "training" then whether you are a solo 24 or a 6+6, you still need to start at 1200 Saturday. You could then take whatever rests you thought necessary throughout the race.
A bit of nostalgia ... In 2006, Rosie Barnes was in training for the UCI World MTB Marathon Championships and she did the Easter Solo 24Hr race. She rode for 100K on Saturday (about 6 hours), had the night off, then rode another 100K on Sunday morning. All up, she did 25 laps and came third overall against riders who rode for the full 24 hours. The Solo female winner only did 32 laps in twice the time that Rosie did 25! Even the winning Elite Male "only" did 47 laps.
Rosie went home with the 7 fastest female laps of the event, and I think she went on to come 23rd in the World Marathon Champs that year.
... it appears the 6 + 6 is the Anti 6-6.....so back to being confused again....
I think the 6 minus 6 category is for people like Red Racer who can start at 1200 Sunday, ride faster than the speed of light and finish at 1200 Saturday.