Here is a bit more complex scenario:
In this case the website uses two SSP and these are (re-)selling to three exchanges. One of the exchanges also (re-)sells the traffic to a fourth exchange. At the end the traffic is offered to several DSP.
In this scenario it is not enough to include only the two SSP in the ads.txt because the DSP will get the traffic from exchanges (indirect). There the DSP will not be able to verify if the (exchanges) partner ID matches the ID in the ads.txt file. Hence it is necessary to include all parties in the ads.txt file:
In short: The website needs to ask both SSP if they are selling the traffic to a reseller. If that is the case, the website needs to add the reseller's info in its ads.txt file (in this case exchanges A, B and C). If one of the resellers sells the traffic to another reseller (exchange A to exchange D), then even this info needs to be included too.
At the end the ads.txt file needs to contain each party which is sending the traffic to a next party, so that the next party can verify it.
Hint: Use our managed ads.txt service in order to automate changes in your resellers ads.txt files!