All and Across the board

All adjective - Not divided or scattered among several areas of interest or concern.

Across the board and all are semantically related in whole topic. In some cases you can use "Across the board" instead a noun "All".

Across the board adverb - Including all.

All and across the board are semantically related in broad in scope topic. Sometimes you can use "All" instead the phrase "Across the board" as an adjective or a noun.