What to display for a self-employed person in the invoice and act for legal entities. faces if I am a web developer?

Question

Law in IT

Good afternoon! I'm starting as a web developer (frontend). The first client appeared and immediately legal. face.
I registered with the tax office as self-employed (NEP).

What should be displayed in the invoice and act as completed work, if the project itself is large, and I periodically perform only some separate part of it?

They say that it is not desirable to indicate hours as a unit of work, so that the tax office does not think about labor relations.

Answer 1

I have concluded a contract with the legal entity for the performance of work according to the assignment. The assignment can be received by phone or e-mail. In the act of completed work, I write this - a list of tasks that I completed. For example, finalizing the loading of wagons, developing integration with accounting, etc.