Anyone can start off a proposal by submitting it from a masternode. The data required consists of ProposalName, ProposalURL, PaymentCount, StartingBlock, PaymentAddress and PaymentAmountUSD. The ProposalURL is a json object with a ShortDescription, LongDescription, PDFURL and any other fields that are required to show users.
What does the StartingBlock property do? Does it determine how long the voting remains open?