Units that are "summon only"

[Ratatwisker]

Hi,

is there any way to set a unit as "summon only", meaning that the unit cannot be bought in shops but only be summoned by a spell (or via script)? For example, the Phoenix and the Infernal Dragon don't, as far as I know, appear in shops, but looking at the .atoms I don't have a clue how this is accomplished.

Any help is appreceiated.