User Exit affect

Dear Experts,

I have a question regarding User Exit.

We have a user exit that populates a field when the data is coming from R/3. The user exit is SAPLRSAP_001 in include ZXRSAU01.The data comes to PSA first and then to Data target.

Should the field get filled up by the time the data reaches PSA or does the field gets populated somwhere before it reaches the data target?

As far as I know it should be done by the time the data reaches PSA.

Am I right? Please correct me if I am wrong.

Thank you.