Portuguese to English Art/Literary History Castles

'1433 - Penela recebe carta de feira franca de São Miguel, por D. Duarte'

My History classes were taken in England, so maybe I'm asking something obvious, but this is a chronology of events with nothing else to help me out.

I've figured that Feira Franca was a medieval festivity, but I can't figure out the 'carta' part.
Some sort of letter or charter?
Does anyone have a clue?

Thanks in advance for your help!