Compton3bravo:
S18 slot allocation has effectively been completed. So if you don't hold a slot for a service at this point, any request would now be considered "ad hoc".
Broadly speaking, scheduled & charter carriers apply for a series of flights for all or part of the season through this allocation process. Exec operators can & sometimes do apply for slots during the allocation process however the nature of their operations means they usually apply days or maybe hours before the flight is due to operate.