DLT_ types live in pcap/dlt.h. They can be requested by the community on a First-Come First-Served basis [i.e. https://tools.ietf.org/html/rfc8126#section-4.4 ] (Although libpcap is not at this time an IETF specification, there have been some as yet-incomplete efforts to do this).
The Tcpdump Group prefers to link to an open specification on the new DLT_ type, but they are available for closed, proprietary projects as well. In that case, a stable email address suffices so that someone who finds an unknown DLT_ type can investigate. We prefer to give out unambiguous numbers, and we try to do it as quickly as possible, but DLT_USERx is available while you wait.
Note that DLT_ types are, in theory, private to the capture mechanism and can in some cases be operating system specific, and so a second set of values, LINKTYPE_ is allocated for actually writing to pcap files. As much as possible going forward, the DLT_ and LINKTYPE_ value are identical, however, this was not always the case. See pcap-common.c.
The LINKTYPE_ values are not exported, but are in pcap-common.c only.
When allocating a new DLT_ value, a corresponding value needs to be added to pcap-common.c. It is not necessary to copy the comments from dlt.h to pcap-common.c.