I have only read the UK DTA but the word "assessable" (assessable or non-assessable) is not found in the document once, it only states what can be "taxed" and by which country, the DTA is one way of claiming funds that are assessed should not be taxed, Thai RD data only comes from CRS, it tells them your remittances, it doesn't say which remittances are accessible or not (CRS doesn't know the detail of the funds behind every remittance, it knows income and remittance values), that will be the point of the tax return and any follow up that Thai RD conduct.