There are 3 basic parity options:

1. Don't send parity bit ("none")
2. Calculate and send parity bit, ("odd","even")
3. Send a fixed value for the parity bit, ("mark"- logical 1, "space"- logical 0)

You can go faster if you don't send the parity bit. Most applications send a calculated parity bit (calculation done by the RS-232 chip) with either odd or even parity.

Sending a fixed value for the parity is a "weird duck". This might be done to essentially "add a 9th data bit", or to force a parity error for some framing reason. When doing that, usually the chip is re-configured just temporarily for a single data byte. So options are "mark","space","none","odd","even". I have no idea why the SAS protocol would send a "mark" for the parity - the rationale for that is protocol dependent. Parity is configured by one of those 5 strings as marto's post explained.