From ASSS Wiki
Extended LVL, or ELVL is a new format for representing maps in SubSpace/Continuum. ASSS is currently the only server capable of making practical use of ELVL files.
Right now the ELVL Format is backwards compatible with the LVL Format. But in the future there may be a switch to this newer format. This will help bring about arbitrary tile types among other things.
They can be generated using a Python script available at the ASSS site, using Continuum Level / Ini Tool, DustEd or DCME.