confused a bit about this. Base mods is what happens when you first install the nod? What is mod options and how does that trigger?

You get the attachment and "attach" it. You get the base mod effect.

You then modify the attachment, with an associated Credit cost and Mechanics check, and you get to apply one of the mod option effects.

you get base mods by installing the attachment. each attachment can be improved further by making a mechanics check which gets more difficult with each additional mod. If an attempt to modify a weapon fails that particular mod can not be attempted again. That being said if you have the same mod multiple times and fail once you can still attempt it for the other instances you just loose one possible application of it.

Ahhhh ok cool thanks!

If it helps, I think that there is a detailed explanation of attachments in the READ THIS FIRST FAQ sticky on this forum.

Edit - Yup, there is.