Show or hide an option based on another choice

The rule

An option only becomes available once something else is selected. A weapon unlocks after a model takes a particular skill, or an upgrade appears only for a certain sub-faction.

The structure

Leave the option visible by default and hide it with a modifier, or the reverse, gated on a condition:

• A set Hidden modifier on the option.
• A condition that tests for the trigger, for example "at least 1 selection of the skill" within the right scope.

When the counted entries are nested below the node the condition runs on, tick "and all child selections" on the condition. Without it the condition only looks one level down and the rule appears not to fire.

Set the condition's scope to the unit or model that should react, not the whole roster. See Scope & Context.

Hide a configuration unit once it's added

For a one-time configuration or "header" entry that should drop out of the add-unit list after it is in the army, set it hidden with a condition that it is already present: an instance-of test on the header childId at Self scope, so the entry only checks itself. Keep it on Self: at any wider scope header matches every header entry in scope, so adding one configuration entry would hide them all. (Testing the entry's own id works too.)

Forbid instead of hide

If the point is that an option is not allowed rather than not yet relevant, consider a max 0 constraint instead of hiding it. The player still sees the entry and understands why it is unavailable, which several established catalogues prefer over an option that silently disappears.