Acumatica allows displaying the endpoint schema with a REST call. This is an example of the Customer endpoint schema.
localhost/acumaticadb/entity/Default/24.200.001/Customer/$adHocSchema
I put the output on my server as it too large to display in a post.
https://johnspikowski.com/acumatica/customer_schema.txt
The Web Services Endpoint screen is another asset for endpoint definitions.
http://localhost/acumaticadb/Main?CompanyID=Demo&ScreenId=SM2070PL