Based on the Wikipedia
In some contexts typical instances of tertiary sources are bibliographies, library catalogs, directories, reading lists and survey articles. Encyclopedias and textbooks are examples of written materials that typically embrace both secondary and tertiary sources, presenting on the one hand commentary and analysis, while on the other attempting to provide a synoptic overview of the material available on the topic.
A different definition is used by the UNISIST model in which secondary sources are understood as bibliographies, while tertiary sources are understood as synthesis of primary literature.
In the table for the article Source Value the term "tertiary source" has been associated with materials such as GedCom, and other ephemeral materials found on the web. Since these sources can not be counted on for long term persistence, are commonly only weakly sourced, categorizing them as "tertiary sources" seems reasonable. This usage is comparable to the UNISIST model usage as described above.