FB70 Customer Invoice

While posting SAP customer invoice (FB70) why system asks for G/L account?
As per double entry book keeping rule:
customer is debited
Customer reconciliation a/c is credited
Why one more G/L account on top of Recon a/c, which is posted automatically?

Solution :
* Entry gets posted to Customer a/c through reconciliation account.
You have to give a GL a/c for revenue.

Your entry would be
Dr.Customer (Reconciliation a/c)
Cr.Revenue

* Reconciliation is a fictious entry so you cannot consider as an entry to be entered by the user.
This rule is derived from the fact that ‘we cannot enter/post directly to RECON account’.

That is why system needs a GL account to make the account balance as zero.
Manual entry could be:
Dr. Customer a/c Dr
Cr. Domestic Sales a/c
(Sales invoiced posted)

Recon entry is automatically made once you post this entry since you have configured your RECON in the IMG.

Also See: What is General Ledger Planning in SAP?