Is sales, what is called month end billing?

Not necessary net sales may include the returns which billing may not be showing.

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Automate month-end close and rid your department of manual process ,also you

Sales is a continuous process , it isn't catch order

Not necessary net sales may include the returns

Invoices are typically marked with a discount period, the net amount due, and some additional information.

An accumulative 30 days usage of a specific service cost.

Your prepay invoice can be added ....

Thanks 

I prefer to wait for more answers from colleagues and experts 

An accounting procedure undertaken at the end of the month to close out the current posting period. It is part of a company's closing operations. Types of accounting procedures addressed in month-end closing can include depreciating fixed assets, reconciling inventory discrepancies, settling work in progress material, posting billing documents, and payroll. See also day-end closing, year-end closing

Yes month end billing is also a part of sales, but it can be for one time in any part of the month. But only month end billing cannot be consider as sales, if you have few sales bills in between the months.