Nominal generally refers to pressure treated wood.
4x4 Nominal fits 3.5" x 3.5" Post Size
5x5 Nominal fits a 4.5" x 4.5" Post Size
6x6 Nominal fits 5.5" x 5.5"
8x8 Nominal fits 7.25" x 7.25"
Actual general refers to rough lumber (not surfaced on 4 sides) or vinyl or composite posts.
Some of these post may still measure between 'Nominal' and 'Actual' sizes, particularly in Cedar material.
A classy Copper top post cap will not only enhance the look of your deck, but it will protect your posts from weathering as well. Available with AZEK, Mahogany, Treated Pine, Redwood, or Western Red Cedar trim.