General Question Help with Attatchments and UMMU

[Submariner]

My goal is to be able to create attatchment points on vessels that UMMUs would be able to grapple on. I could use this to simulate UMMUs performing repairs on the ISS exterior, for example. Does anyone know how to do this? I have been looking for information on how attachments work, there seem to be different types. I've seen the letters XS, GS and OS after attachment definitions in config files. What do these mean? And UMMUs need a special type of attachment point to be able to grapple to? I've seen attachment points with UCGOCG after them in UCGO cargo configs. Thanks in advance!