The Nominal Size of a filter is the rounded size of the filter. This is usually the size that is displayed on your filter.
Ex. 20x25x5 (19-1/2 x 24-1/2 x 4-7/8)
The first number in this example is the Nominal Size of the filter.
The Actual Size would be the exact measurements of your filter. If looking on your filter, these measurements are typically housed within parenthesis.
Ex. 16x25x5 (15-1/2 x 24-1/2 x 4-7/8)
The second set of numbers in this example is the Actual Size of the filter.
The Form Factor is the style of filter you might be looking for.
Ex. Expandable - An accordion style filter that expands to fit the application.
There are several different types of filters for various applications.
If you're looking for a filter with a special feature, like odor absorbing, that would be located here.
You can use these feature to search by Model Number for the filter you require.