This unit is basically a wrapper for the old style init script that will attempt to configure all DRBD resources as defined in the global configuration files.
It can not be parameterized, and there is no useful way to wait for any or all DRBD resources to become "ready".
You should usually disable this unit.
If you use a cluster manager like pacemaker, that should bring up the DRBD resources instead, and via the DRBD ocf resource agent integration also notices when DRBD is "ready" to be used.
If you use linstor and/or drbd-reactor, linstor will bring up the resources, and drbd-reactor will attempt to bring up the services once DRBD becomes ready.
If you want to "hardcode" a promotion/mount attempt of specific DRBD resources into your boot sequence, you should be using drbd@RESNAME.target (see there) and drbd-wait-promotable@RESNAME.service, implicitly drbdRESNAME.service, and maybe optionally drbd-lvchange@RESNAME.service.
If you think you want to enable this unit, you probably still should disable this unit, as this unit may not do what you think it does, and there likely are better ways to do what you expected this unit to do.