These brackets, in conjunction with the extension table rods that come with the Model 520 table system, allow the use of the extension tables directly in front of or behind the main table, for infeed or outfeed support. You should still use an extension table leg with these.
The “Front” version has enough space to permit operation of the 520 fence so long as the extension table is not directly in front of the fence. The “rear” version is shorter for increased rigidity.
You can double them up to hold the extension table relatively flat.

With the front brackets, if you're not running a larger printer such as a Prusa XL, recommend printing left and right side of the brackets separately, you'll still have to fiddle with placement and orientation, but it should fit on the print bed, even if you can't place it flat.

For printing, this is a structural piece. Recommend increasing vertical shells for strength.