The RS version is designed to permit use in a replenishment regime, and the non-RS version isn't.

The non-RS version has problems with dichroic fog when used with sheet film, and Kodak recommends against using it with sheet film. The RS version is recommended for use with sheet film.

The data sheet is fairly informative:

"T-MAX Developer is intended for use in unreplenished systems. For replenished systems, use T-MAX RS Developer and Replenisher. T-MAX RS Developer and Replenisher is a hydroquinone-based, two-part developer specially formulated for replenished systems, but you can also use it in unreplenished systems.
T-MAX Developer is available as a one-part concentrate in sizes to make one gallon and five gallons of working solution. You can easily mix smaller volumes by mixing one part of the concentrate with four parts water. T-MAX RS Developer and Replenisher is available in convenient sizes to make one gallon and ten gallons of solution; use this solution as a working-tank solution or a replenisher. The ten-gallon size consists of two separate units, each to make five gallons of solution."