From my knowledge, for ISEL it's just the order your package was reviewed/accepted. The spots are filled chronologically based on availability.
For the regular board there's a few more nuances. For example, if someone is near the age limit or has an age waiver- they will get priority in receiving an OCS date. For boards, like supply they were assigning dates alphabetically by last name. Then some applicants like BDCP can't ship until they graduate. Tons of different scenarios.