You can extend the capabilities and functionality of AIS by enabling it to invoke code that you write. Among other areas this includes GDBs, UDTs, Adapters, and Procedure Drivers. Some features are enabled by the addon file and others by XML attributes in their definitions. For features enabled by the addon file there is Appendix 17 in the ac_sdk.pdf document detailing platform specific notes and limitations. For OpenVMS it states that the DLL attribute value must be a logical containing the full path of the shareable image. (What OpenVMS calls a DLL.) While that is one (and the first) way that the DLL can be specified it can be specified in 2 other ways on OpenVMS:

If the logical or file specified by the logical is not found, then AIS will attempt to load the DLL from SYS$SHARE. If not found in SYS$SHARE, it will attempt to load the DLL from NAVROOT:[BIN]. If not found in any of those 3 places (via logical, SYS$SHARE, or NAVROOT:[BIN]) an error is reported.

This information is applicable for any AIS extension on OpenVMS whether it is by means of the addon file or XML meta data.