The ID states if it can use a liner. Some of the bretonian IDs, for example, have "Royal Liner" added to the list of allowed ships. If the liner is not listed, then you can't use it.
It's considered a civilian (open to all) vessel. The restriction on liners is largely by ship class however; there are only a few factions that are allowed to use the "Liner" class vessels.
Liners are not transport class ships.
Wide awake in a world that sleeps, enduring thoughts, enduring scenes. The knowledge of what is yet to come.
From a time when all seems lost, from a dead man to a world, without restraint, unafraid and free.
Mostly retired Discovery member. May still visit from time to time.
I've also seen one or two OSC IDed Slave Liners, that RP it as 'Old' liners put in use for second-class tours. That's completely ok as the OSC Id allows any type of Liners.
So, to use Slave Liner, you should either get one of the IDs that allow "Liners" or one of the IDs that specifically allow "Slave Liner".