The Inner Core Baffle is the perforated, cylindrical, stainless steel tube inside the exhaust can. Working from the outside in, there's the exhaust can skin, then the pillow, then the inner core baffle. Note that this is not the quiet core, but a different part entirely.
Baffles are sold by length. Most Barker's mufflers are either 12" or 14", measured end to end, not including the exhaust tip. The RZR900S and the RZRXP1000 use an 18" baffle. Please measure your muffler to ensure you are purchasing the correct size baffle.