If you have four leaders signed up on missions any choose not to do one of the missions with 1 leaders is that is that considered passing or to put it more simply would you have to pass and only react with the leader is left in your leaders pool

Not sure what you're asking. You can choose not to do any of the missions you have selected in the assignment phase during the combat phase. If you "pass" you are claiming that you are done for the entire combat phase and will not be able to perform any more tasks with your leaders. Unused mission cards go back into your hand during clean up. Leaders on those cards are not placed back into the leader pool until the cleanup phase as well.

Lets say you put Vader on a capture mission but the Rebel player either didn't play any leaders in a system that contains an Imperial unit, or he did but the leader is not one he wants to capture. You (the Imperial player) can do other actions but once all there is left to do is reveal the capture mission, you can pass without revealing the mission, and so the mission will go back into your hand at the beginning of the Refresh phase.

You are not required to reveal missions in any particular order. In fact, you are not required to reveal missions at all.

During the Command Phase, both sides take alternating turns performing one of the following two actions: 1.) Reveal a Mission and 2.) Activate a System.

Each time it is your turn during the Command Phase you must take one of those two actions or you must pass. If you pass, you can no longer take one of those two actions during the current Command Phase. If any of your assigned missions remain unplayed after you have passed, later during the Refresh Phase you will return those missions to your hand and retrieve the leader(s) you assigned to the missions to your leader pool.

Thank you for clearing that up I appreciate it I was using voice to text so we got lost in translation LOL