Nominal generally refers to pressure treated wood.

4x4 Nominal fits 3.5" x 3.5" up to 3.75" x 3.75"
(The aluminum caps only fit 3.5" x 3.5")

6x6 Nominal fits 5.5" x 5.5" up to 5.75" x 5.75"
(The aluminum caps only fit 5.5" x 5.5")

8x8 Nominal fits 7.375" x 7.375" up to 7.625" x 7.625"

Actual generally refers to rough, untreated wood and vinyl or composite posts.

4x4 Actual fits 4" x 4" up to 4.6" x 4.6"
(The aluminum caps only fit 4" x 4")

6x6 Actual fits 6" x 6" up to 6.25" x 6.25"
(The aluminum caps only fit 6" x 6")

8x8 Actual fits 7.875" x 7.875" up 8.125" x 8.125"
(The aluminum caps only fit 8" x 8")