I recently found a Relic slot item that says "Use: Increases Critical Rating and Surge Rating by 50 for 20 seconds."
Critical Rating deals with Critical Hits, but what is Surge Rating?
Surge Rating
star-wars-the-old-republic
Related Solutions
Rating is the quality of the item (explained here under item rating)
Equippable items have a rating indicating their general level of quality. This number gives you a rough idea of how powerful the viewed item is compared to other items you’re wearing. To raise the rating, look for certain modification items that can increase these ratings, and attach them to your equipment.
What you pay to get an item to an auction is a deposit. And you get it back when the auction ends, whether successfully or not. If the item is sold, a commission is deduced from the credits you receive.
For a two-day auction, the deposit is 15 % of the asking price. The commission is 3 % of the price (Source: my own calculations based on 4 successful sales.)
For example, in the sale from the screenshot below, the asking price was 1800 credits, the deposit was 15 % of that, or 270 credits (it was a two-day auction) and the commision was 3 % of the price, or 54 credits. So, I received 1800 + 270 - 54 = 2016 credits.
Best Answer
Both of them are related to critical hits. Critical Rating affects how often do you make them, Surge Rating affects how much damage do they deal if you make them.
For details, hover over Critical Chance and Critical Multiplier on your character sheet: