MDB uses CTF debug information to read and display structures correctly. CTF (Compact C Type Format) is a reduced form of debug information similar to DWARF and stab. CTF describes types (structures, unions, and typedefs, for example) and function prototypes. Solaris kernel binaries embed CTF data as an ELF section (.SUNW_ctf).
As much as possible, CTF data is stored in one place to minimize duplication of common types. Other occurrences of each type reference the one unique definition. When a kernel update is released, existing CTF structure definitions must be preserved because some kernel modules might not be updated and might still be using the old definitions. When a kernel update is released but not all modules are updated, the CTF definitions are held in the module in which they are defined. When you use MDB to examine a crash dump, you might see a message that the structure you want to examine does not exist, or you might see an indication that the structure has changed. If the structure definition has changed, the data might look corrupt, for example. If you encounter either of these conditions, use the scoping operator to specify the module where the structure is defined.