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.
Deck post sleeves enhance the look and feel of your deck