It is important that extension producers understand how reference sets and their members are created. All extensions must include new members in the 
to define the modules on which the extension module(s) depend. Many extensions will also specify language or dialect preferences in a 
. Some extensions may also include additional reference sets to extend the use of SNOMED CT. For example, a subset of concepts for use in a specific clinical setting can be distributed as a 
.
The following subsections present the logical design rules and considerations for reference sets and reference set members in an extension:
