Apologies, but how does "attach to this unit" work?

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CraftyScreen

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How does "attach to this unit" work?

I looked for it on forums, but "Search this Forum" tells me the words are too common to search on, and putting them all in quotes doesn't help.

Different posts in random places say that "attach to this unit" either:

a. attaches your unit to allied units
b. tells your allies to attach their units to yours.

It can't be both, right? Does it mean one thing in CK2 and another in EU4?

If I have two stacks leaving a province, does "attach to this unit" tell the allied troops to follow that one?

The AI's behavior hasn't been that clear. Sometimes all I need to do to get allied troops moving is go visit them with some horsemen. Other times they just sort of wander around. Nice to have the company, but sometimes they get themselves clobbered, and I think that's bad for warscore.

Is there an canonical explanation somewhere? Thanks! And my apologies for the n00bie question.