Squaring up is very different from just plain measuring. On this ruler, the numbering goes from one corner along two sides. It does not go both left and right.
The entire ruler is gridded using the same symbols as Marti Michell's two larger rulers, but in addition, there is a shadow that follows every inch mark, turns a corner and connects to make seeing squares very easy.
Right-handers generally put the focal corner in the upper right while left-handed cutters will position and measure from the upper left.
When those two exposed edges are trimmed, the fabric piece is rotated so that the trimmed sides line up with the correct measurement and the untrimmed sides are ready to be perfected.
There is one 45 degree diagonal line. Many people like to use that when they square up half-square triangle pairs.
The last 1/4" on two sides is used to check the accuracy of seam allowances.