Domain Authority (DA) is a measurement of how well your site ranks on search engines like Google for an industry or topic of interest relative to the amount of total competition. Page Authority ¶ is a measurement of how well your website’s webpage ranks on search engines like Google for an industry or topic of interest relative to the amount of total competition.
So since DA measures rankings about all other sites and PA measures rankings about only one page, they are different but rely upon each other.