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There is one custom field, but I don't know on where that field value is set. What can set this custom field value? Suitescript / Workflow or what? How can I check?

Check the defaulting subtab of the field configuration

Then check the sourcing subtab

Then try scripted records for that record type and look for any workflows/scripts deployed to it

Review each workflow and review the actions to see if any set field value actions relate to that field

Lastly get the field ID and look in all the script files deployed to that record and use ctrl + F to find the field ID

You could also use the Flashlight plugin by strongpoint. I think that covers your exact use case.