DRM meaning in Space Science ?

The NASA Mars Design Reference Mission ("DRM") refer to a series of NASA conceptual design studies of the missions to send humans to Mars. The related term, Design Reference Architecture (DRA), refers to the entire sequences of missions and supporting infrastructure.

These are a reference baseline studies summarizing the current technology and possible approaches for a human mission to Mars, and are not actual mission program. According to NASA, the documents "represent a 'snapshot' of work in progress in support of planning for future human exploration of the Martian surface." The design reference missions are used for technology trade studies, to analyze the effect of different approaches to the mission.