Hi, Anna,
I can help with two of them:
X2 - isn't an acronym but rather he was just saying that he originally thought one person posted before him and suddenly there were two post so he acknowledged that saying ""X2" or "times two".
POP - is a wonderful acronym that came over from PF and, I believe, originally coined by Ben Powell. It's "Pool Owner Patience" and can be applied to a variety of situations but primarily refers to having the patience you need when clearing a green, neglected pool.
After you do the testing, put in the chlorine, turn on the pump, etc. you then take a big dose of POP because it often takes a week or so to get your pool clean. As you will see this Summer, there will be posters who want months of neglect to be resolved in 24 hours.......they really need POP but usually go to the pool store to purchase the "magic bullet" that will solve all their problems. Ooops, I digress....
ETA - I didn't see it in the thread but military and aviation folks use it a lot for Estimated Time of Arrival......don't know if that was the case here or not.
Anna, I had an opportunity to explore your website about your pool and all the things you've done yourselves. Solar covar, skimmer access, etc. Wow! ya'll are very clever and your workmanship is really nice. I hope you'll keep referring folks over there.....ya'll have a lot of good ideas from which members can benefit. Your computer skills in putting it all together are very impressive as well. One of the most pleasant sites I've visited.......very helpful. Thanks!!