The current version of the LEMS specification is 0.7.6 and the schema for this can be seen here.
The following figure, taken from Cannon et al. 2014 ([CGC+14]) shows the structure of LEMS models.
The following pages give details of all the elements that are included in LEMS.
For examples on LEMS, and using LEMS to extend NeuroML, please see the relevant sections in the documentation.