Taxation Ordinaire Ulterieure

Hey everyone, I arrived here in November 7th 2023, in Vaud, from an EU country. I was working for a company in my home country and kept on working for them until the 31st December 2023. I was taxed at source back in my home country, and then in January I started a contract here in Switzerland.

I was recently told that I need to submit a TOU document before March 31st. I checked and it seems like as I was earning money from another country, I need to submit this document. I filled it indicating how much I was taxed at source. I have both my pay slips from November and December attached. But now I was told by another person that this is not necessary to submit this. I will still send it because I read the rules, but can someone please tell me what this entails and if I should be aware of something? I am leaving in vacations next Wednesday and nervous because I only come back in April 1st, leaving me no time to correct any possible mistakes.