26) Your income in Thailand is defined as any money paid to you inside Thailand, as well as, potentially, any money you receive from overseas. Both types are potentially assessable income for Thai Tax Residents. There are many types of income that can be classed as assessable. The Thai RD lists some of them in Section 40, 1-8, excluding that income that is excluded by a DTA or other RD rules, and is linked below. However, the list is not exhaustive and does not consider all of the many different types of overseas income that foreigners may have:
28) In Thai tax law, virtually all income is assessable, unless it is subsequently excluded by the taxpayer or under Thai RD rules - there is no concept of non-assessable income. Income is understood to mean money that is received from any source. The definition of income that is derived from within Thailand is fairly clear, if you work and have a job and you are a Tax Resident, the payment you receive is assessable for tax. Interest that is paid to you on Thai bank accounts is regarded as assessable income, as is income from investments such as stocks and bonds within Thailand. As a general principle, any payment you receive for work that arises within Thailand is regarded as assessable income. You should note that if you are generating income by working while staying in Thailand, it is (and has always been) irrelevant where that money is paid and whether you bring the money into the country or keep it offshore. That money arises in Thailand hence it is taxable here.