When/what/how Minimum limit of TDS needs to be considered

Hi

I would like to know whether the minimum exemption limit (say for Sec 194A, it is 5000/- or 194C 20,000/-)  is per vendor and per annum or per transaction?

And also, whether the same rule is applicable if it is TDS calculation is at the time of payment or invoice(ie credit)?

Thanks

sri