Captain-major is the English rendering of the Portuguese title Capitão-mor (sometimes Capitão Donatário) for colonial officers, put in charge of a capitania, Portuguese possession (called captaincy, after his title) deemed not (yet) important enough to have its own colonial Governor.
Due to the impossibility of exercising direct control and sovereignty over islands, the captain-major was the channel by which the sovereign could delegate his powers, with certain restrictions, under the responsibility of peoples he felt he could confide. The Donatário could administer, in the sovereign's name, the lands for which he was assigned, with all the regalia, rights and obligations, with the exception of certain limits, including military authority over soldiers and detachments of the crown, or the administration of justice.
In addition to the regalia of office, the captains-major would hold title to the closest parcels of lands, be able to contract renters of these lands, in the name of the crown. Many of the donatários were hereditary, with a few exceptions, referred in the Salic Laws (Leis Sálicas) of the time. The Salic laws regulated all aspects of life, including crime, taxation, indemnity, and female inheritance.
This was notably the case in the following Portuguese colonies, in this incomplete list:
Read more about Captain-major: North Atlantic, Africa, South America, Asia, Captains, Other Meanings