Help with deciphering shorthand on workouts (USMS Open Water ones)?

Greetings! Welcome to the wacky world of swimming shorthand, subject to regional "dialects".

EZ just means casual, not pushing it. It might be 50% for you, or even 30%. And what is easy-going for you might not be the same for another swimmer. It's not meant to be a standard of any sort. 

"P500" is a new one for me, but my best guess is that you are meant to keep to what your pace is for a 500 while doing the 50 (meters or yards). So each rep, 1-5 (of 50), would be at an effort level of your cruise pace for 500 (meters or yards). So the main set is presented for you to alternate pushing it then relax.  Intervals of 50 are typically applied to sprinting, but as it would apply to open water training, this is a way to get your heart rate up while helping you work on breathing rhythm and maintaining a pace, both key concepts with long distance swimming.  

I'm interested in what the rest of the workout says to do with the remaining 50s (6-20). 

Going on with the 14x50 @ 1:00 1, 2, 3, 4 at 200P, EZ recovery after each:  That would be fourteen 50s (meters or yards) with each 50 done in 1 minute. As noted like this, your rest is whatever is leftover in each minute (so in actuality, each 50 might be really fast, just so you can have any rest at all). The 1, 2, 3, 4 at 200P refers to the first 4 50s and I would expect it is looking for you to stick to a pace (for a 50) that is equivalent to what your 200 (meter or yard) pace is. This is likely going to be slower than your cruise pace for a 500 because you typically set into a good pace the longer you swim (when training for long distance vs. sprinting). 

How the person who wrote this has decided that you should add an extra unknown length after each 50 that is "EZ" (easy going) is a little unusual. It would be better to note it separate. Alternately it could be listed as 14X100 broken (with directions to slow for the second half of each 100).  In essence you would be doing a total of twenty-eight 50s (1400 meters or yards). 

Does that help at all or made it more confusing? 

I'm curious if this workout presented as open water training in a pool? 

Here are some additional resources that might be of help to you: 

Terminology does vary somewhat across regions and LMSCs, even levels outside of USMS, but here is a reference through USMS for some terminology: https://www.usms.org/workout-library/glossary-of-swimming-terms?gclid=EAIaIQobChMIs_SDjtzL9AIVlq_ICh0DlAN5EAAYASAAEgLnbvD_BwE

Coach Elizabeth

Been a while so it isn't fresh on my mind.  I think I worded it poorly.  Let me throw some numbers at it.  My "easy" pace during sets is about :40, the 500 pace I've been targeting is :36.  I'm wondering if I'm supposed to go 36, 40, 36, 40.......etc. for 10 reps.  WHich only gets me halfway.  Or am I supposed to go 36, 36, 36, 36, 36, 40.  Which is 6, but if I do an easy after every 5 it kind of fits the 20X50 seqeuence, but I'd do 24.  Not really sure, I was used to doing either 40X50 on :55 for 500 pacing (20 seconds rest) based on workouts that were posted when I started swimming.  Now I usually just do Hackett sets with my base interval at :45, and I come in around :36-37 for the slow, and :34-35 for the fast.

Anyway, thanks for replying, I had forgotten about this. I posted up this I think when I was just getting into tapering down for my 10K open water.  Not really necessary to sort it out, now.