As described in Section A.1.4, unit codes may be defined locally. These codes all begin with bits 29 to 31 as 111 (form 7). The specification of the other bits is left to each site to define. This allows for sites to define special units that will not fit in any other categories. The intention is that locally defined units codes will be used as a temporary measure while registration of an equivalent centrally-defined units code is pending. Also, these will not have to be registered through the central site, allowing for flexibility on the part of each site.