POsting routine

AST

Hello, I wrote this question previously in the wrong forum.
Could anybody explain me what exactly happens when I post document (e.g Purchase Invoice using form not journal)? I know that purchase documents are posted by codeunit Purch-Post (90) but do I have only one line in journals for one line in Purchase Line? How many lines do I have in General Journal when I choose "Payment methode code" with "Bal. account type" not G/L Account? What is the role of Invoice Post. Buffer table?
I'm a little confused about this.
I use Navision 4.0 with SP2.
Is there any document describing this process?

Thanks Deadlizard for your answer, but I need more information? Where can I find this document?

kine

Invoice Post. Buffer is used to "Group" the GL entries by posting groups etc. to post as few entries as possible (they are grouped by Type,G/L Account,Gen. Bus. Posting Group,Gen. Prod. Posting Group,VAT Bus. Posting Group,VAT Prod. Posting Group,Tax Area Code,Tax Group Code,Tax Liable,Use Tax,Dimension Entry No.,Job No.,Fixed Asset Line No.).

The CU 90 is used to post the document, but the CU 90 is using CU 12, 22 etc. to post relevant data into G/L entries, Item entries, Resource entr. etc.