Pilot number is a number that only exists to control another number or function. So a hunt group has a pilot number of ,s ay, 1000 and members of , say, 1001, 1002 1003 etc. You dial the pilot number to ring the whole group. The pilot number may also be a real DN which has other numbers in the group.
With multicall its the same except the pilot number is a single number which is repeated across a number of handsets using keys. Ring the pilot number and all the keys ring on every phone they are provided on. The pilot number can be a real DN on a handset or a DN that only exists to be the pilot number of the multicall group.
Could also be a personal ring group? What are the night service settings in call rerouting for the DN that is diverted in night service?