Common registers
From Unofficial QEdit Wiki Guide
For common functions that these registers may be seen to be use in see common_functions
Common Registers
The following registers are reserved and must be used for those purposes.
R0 = Says the quest has begun. R74 = Used to hide show quest board item 0. (Set true. Clear false.) R75 = Used to hide show quest board item 1. (Set true. Clear false.) R76 = Used to hide show quest board item 2. (Set true. Clear false.) R77 = Used to hide show quest board item 3. (Set true. Clear false.) R78 = Used to hide show quest board item 4. (Set true. Clear false.) R250 = Player slot number. R253 = Is the quest failure flag. (If set quest failed.) R255 = Is the quest success flag. (If set quest cleared.)
For the rest of the registers you can mostly use any register for your purposes, here are some common registers and their use for your help.
R251 = Number of players R252 = Difficulty R254 = quest win (If true, reward is available)