In a refreshing example of the Upper Tribunal choosing not to lay down wide-ranging obiter guidance on limited facts having not heard relevant submissions, the headnote for Fatima (paragraph 1 (d) Appendix FM-SE: interpretation)  UKUT 155 (IAC) says simply:
Interpretation of the phrase “lawfully derived” in paragraph 1 (d) of Appendix FM-SE is to be on a case-to-case basis. The drafter of the Rule did not see fit to provide a definition, and it would not be appropriate for the Tribunal to do so.
The sponsor in this case had worked for an employer who had, unbeknownst to the sponsor, failed to make national insurance and tax contributions. This did not stop the income being lawfully derived, as “his income arose perfectly lawfully from lawful employment”.