You use the same AI editor, but it's not an AI (so it's stored somewhere else, for example in a small drop down thing). Condition and Connector nodes don't change, but there's only one action: Select<Target selection; Filters>. Subtrees are not available, since you can even select a custom target selection in the select action. You have to specify the target type of this custom target selection "blueprint", since for example resources don't have shields.
If there are no valid select nodes in the "blueprint", I think the action node which called this custom target selection thing should become invalid. This idea will make debugging harder, that's kinda a downsight. I know GFX47 plans on creating pre-built actions, or something like that (Attack weakest enemy), this is something similar probably, but obviously with more freedom. This is better than subtrees because you can still use the filters here, which can be used as an input arguments (you don't have to make a AttackWeakestEnemy - ShortRange; ... - MidRange; ... - LongRange subtree, instead you can just make a custom target selection and use filters).