Issue = # of notes released by the central bank into circulation (for honest monetary use)
Specimens (code "s" at the end of the P#) = educational notes (often used for bank employee training) & not issued/no value
The varieties for some denominations are significant for some collectors (esp who may specialize in Spanish notes). This 1 Peseta (P-144 example) had a half billion issued so the varieties may not be sought after as avidly compared to tougher higher denominations (by the majority of World paper collectors).
Spain usually issued notes like this:
First series issued: (no prefix or no letter preceding the 7 digit serial number- just the serial number) that's P-144a.1 (most uncommon/toughest variety)
2nd series issued: (prefix, or letter preceding the 7 digit serial #) that's P-144a.2 (common) & the example used here on Numista
3rd series issued: (number + prefix + 7 digit serial #) that's P-144a.3 that's the example used on the Banknote Museum note (very common). Typically the last issue is the most common if it has a normal run like the 2nd since more collectors catch these for their sets.
Be patient with yourself when learning this numismatic stuff. Take your time (it can get overwhelming) but believe me most of these terms (& varieties) do mean something to most collectors. About 5 years ago, I created a Notable Terms page to help collectors learn some of the typical jargon. Check it out, it may help clear up some questions.