Tax invoice in civil departments....

I have a query regarding payment made to contractors in governmental civil departments (PWD, I&WD etc).

The work alloted to contractors are done under the supervision of engineers and they record all the measurement in measurement book. And after work is done they prepare bill for payment to agency.

So, does engineers require tax invoice from contractors before making payment?

Because whatever gst deductions are there engineers know and they deduct 2% from that.

So what is the role of tax invoice from civil departments point of view?

What do the engineers do with the contractors tax invoice.

Please advise me.

Thanks and regards