You can specify grid units in page, map, or relative units. There are three page units: millimeters, centimeters, and inches. If any of these units is chosen, the distance calculated for the grid component is on the page, not in real-world lengths. The grid tools calculate the length of page units using the scale of the grid. For example, using 2 centimeters as a distance, interval, or length in a grid tool will result in a feature that has a distance calculated as 1,000 meters on the ground if the grid has a scale of 1:50,000 (2 * 0.01 * 50000 = 1000).
Map units are divided into two categories: linear/projected or angular/geographic. Linear unit options include feet, yards, miles, nautical miles, meters, and kilometers. Linear units are displayed when a grid component is defined using a projected coordinate system. Because a grid's primary coordinate system must be a projected coordinate system, the default units listed will be linear.
If an ancillary coordinate system is chosen, which is also a projected coordinate system, the units remain linear. Some grid components, like the mask, cannot have an ancillary coordinate system specified, so by default, they inherit only linear units from the primary coordinate system. Any grid component that requires a distance, interval, or length using linear map units has an actual real-world distance calculated. For example, a value of 200 meters creates features at 200 meters on the ground in the feature class.
Angular map units
Angular map units include degrees, minutes, and seconds. Angular units are only available when the ancillary coordinate system chosen is a geographic coordinate system. Any grid component that requires a distance, interval, or length using angular units has an actual real-world distance calculated using angular measurements. For example, a value of 2 minutes creates features at 2 minutes on the ground. The actual distance on a map changes based on the latitude and longitude of the map's area of interest.
Relative units include a single option called Percentage. This option is only available on grid lines, ticks, and points. Instead of describing an absolute distance or location on the ground, Percentage looks at the length or distance of the source feature or component and divides it accordingly. For example, if a percentage value between 33.3 and 50 is used, only two sets of points are created. Values greater than 50 result in one set of points being created in the center, and values of 33.3 or less result in the creation of two or more sets of points.