It’s essentially the same thing. We offer either a one-off fixed cost, or we agree with you the minimum requirements to be delivered over the course of a period of time (typically a year). This means we can guarantee someone will be available when you need it.
Holiday cover can be booked in advance, whether on-site or remote, but emergency cover can also be accommodated at short notice for an open-ended period of time (e.g. for sickness, or while you hire the right person to fill a role).