ATM I ran some tests, but I can't figure out what Mana Loss is for (I had 2 thoughts what could it be for, but both proved wrong)
However I found out, that Purge spell is tightly tied to the other purge version (both are under Orc - Units spell tab). To test this, I gave one Purge version an editor suffix - Orc Shamans use this one; however I edited values in the one without any suffix - duration and move speed update frequency, and it did have effect in game.
About Purge slow effect
When you use purge, that unit's movement speed will be reduced to 150. That unit's movement speed will start returning to its default value in the last 1/5 of that spells duration - in other words, if Purge duration is 15 seconds, move speed will return to normal over 3 seconds. If duration is 5 seconds, it will return over 1 second.
About Move Speed Update Frequency:
This shows how many times movement speed will be updated, when returning to its default value. If it is 5, it will update only once, if it is less than 5, it will not update at all; if it is 50, it will update 10 times, if 100, then 20 times.
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I didn't test Attack Update Frequency, but I guess it works the same as Move speed frequency.