The purpose of the declaration is to inform SARS of the change in tax residency that will impact the basis on which you will be subject to tax in South Africa and how your returns will be assessed going forward.
Persons who are tax resident in the Republic are taxed on their worldwide income, subject to certain exclusions. Persons who are not tax resident are only taxed on their income from a source within the Republic.
Furthermore, when a tax resident ceases to be a SA tax resident there is a deemed disposal of worldwide assets held by the tax resident (subject to certain exclusions) on the day before that person cease to be a tax resident for an amount equal to the market value of the asset on that date. Therefore, a person should cease tax residency timeously to ensure that offshore assets accumulated after emigration does not accidently fall within the SA tax net.