Your question inclined more towards the functional aspect of Navision wherein the functional/Application consultant already know the functional process and how the entries for that process is created .
The same thing is applied in Navision and posting routine are written on the basis of functional process followed in various domains including taxation .
Hereby I suggest start reading functional manual of Navision and start following execrise given in the manual and Use 'Navigate' function of Navision to understand the entries created for every transactions.
And what does adding a text field to the sales header have to with this question?
The best way is to check Mark Brummel's video :