Max Units (in Resource Sheet) is the availability of that resource on today’s date. This value is calculated from the “Resource Availability” table in Resource Information dialog. It doesn’t consider if the resource is occupied with work or not.
Resource Availability (in Resource Information) indicates the availability of the resource during a particular period. Say 23 Sep 2020 to 23 October 2020 the availability units is 100%. If today’s date is 22 September 2020, Max Units will be 0%. If today’s date is any date between 23 Sep 2020 to 23 October 2020, the Max Units will be 100%.