What is the purpose of sales post codeunit
The purpose of the sales post codeunit (no. 80) is to take your sales header/lines (order, return-order, invoice or credit memo) and turn it into posted documents (i.e. sales invoice header/lines) and the corresponding general/customer/item ledger entries/value entries etc.
how 80 and 81 code units are related
If you check the code of codeunit 81, then you’ll see that the “only” purpose of this codeunit is to have a user interface to running codeunit 80.