RO and RO/DI units typically have a sediment filter, 1 Micron or 5 Microns. The DI I have (Kent Deion 2000) has one carbon block filter and 2 DI chambers, no micron. I suppose one could add on a sediment prefilter if they wished.
On our RO/DI at the shop, for a while we ran a second 1 Micron prefilter because we were going through a lot of cartridges - the water was kind of "muddy" coming in. That seems to have resolve so we are back to one. Our incoming TDS is about 35 right now (when I checked this morning). I've seen it as high as 272 and it will change depending on a variety of factors.
On a RO/DI, or RO, keeping that sediment filter changed out regularly, along with the carbon block, can help prolong the life of the membrane (the expensive part!) The more stuff that gets pulled out before it gets to the membrane, the better. I like clear canisters on my RO/DI so I can *see* the sediment filter without opening the unit up. When it starts looking dingy, we change it.
Jenn