extensive vs across-the-board

extensive and across-the-board both are adjectives.

Word	Noun	Adjective	Verb	Adverb
extensive	No	Yes	No	No
across-the-board	No	Yes	No	No

As adjectives, extensive and across-the-board are synonyms defined as:

Other synonyms of extensive include all-embracing, all-encompassing, all-inclusive, blanket, broad, encompassing, panoptic, wide.

extensive (adjective)	across-the-board (adjective)
of agriculture; increasing productivity by using large areas with minimal outlay and labor	broad in scope or content
broad in scope or content
large in spatial extent or range or scope or quantity