At this time, we have only a few documents with information on this subject:

• The X Session Manager (XSM)
  

  An introduction to what the XDM provides, and what its limitations are.

• XDM Loop-O-Death Fix
  

  A proposed method proposed to eliminate the possiblity of XDM respawning a broken/misconfigured X server from here to the end of time. The thread continues to present several other alternatives, including a patch to the source itself.

Documentation