Now you are getting a bit technical for me :-)

In the paper form you will see the races that have emergecies at the bottom of the entries. I guess they limit the starters for safety reasons on various tracks. If there are scratchings the emergencies get the chance. Who gets the call up I do not know.... Probably ballotted in some way as I guess is done in the first place when entries are closed and there are too many starters.