US

Check this if you want to use United States measurements. Dimensions will be shown in US standard feet' and inches". For example: 2' 6" or 3' 5 1/4"

Metric

Check this if you want to use metric decimal notation. Measurements will be shown in meters with fractions representing centimeter and millimeters. For example:  1.23 equals 1 meter and 23 centimeters.

Nudge Amount

This lets you set the nudge amount used when you select objects, hold down the Ctrl key and press one of the arrow keys to nudge it. Settings are in centimeters for metric and inches for US.

The default is 1/10th of an inch or .25 centimeters.

Grid Spacing

Sets grid line spacing in both blueprint and 3D viewing modes. Settings are in centimeters for metric and inches for US.

Show Fractions with US Measurements

Show inch fractions with US measurements down to 1/8th inch.

For example, if this set you can position windows 2' 3 1/8" inches  apart. If not set then the distance would show as 2' 3". The dimension is rounded to the closest integer value. For general design work the fractions get in the way. For detailed construction work they are important.