There are three dimensions on saw chain.
The distance between tips of the drive links divided by two. I think this is the 3/8" that you're reading.
There is the thickness of the drive link which corresponds to the rail slot on the bar. Generally, .050 or in that range.
Then there is the dimension of the cutting edge. This is the one that you size the file too.
If you can't decipher which number you're looking for you can add whatever info you find in the owner's manual. Most of the time the info is stamped on the side of the bar too.