I found some definitions in freevms-0_3_15.gz, in particular ihddef.h for the header and isddef.h for the sections. There is also some code which uses those structs to parse the executables:
struct _isd * section=(unsigned long)buffer+ehdr32->ihd$w_size;
long symtab=0, symtabsize=0, symtabvbn=0, symstr=0, symstrsize=0, symstrvbn=0;
The following call stack can be found in MrmCoreR.dll:
Seems like it's used by Metro applications to know if they were run in the sandbox. And RtlCheckSandboxedToken() checks the token if it can be accessed via some specific ...