It is typically the most specific description encompassing the set, so for instance a set including lion and moth designs might have the design type "animals", while a set with "eland" and "wildebeest" might be "mammals", "big game", or "African wildlife".

The design type is necessarily somewhat subjective. Modern postal authorities frequently publish an official intent for the design type, while for older stamps the description is based on philatelists' consensus.

There is some overlap with the notion of stamp issue. However, the issue is frequently a description of the intent or purpose of the stamp, such as to mark the 100th anniversary of an organization, while the design type summarizes the common theme of several different designs - perhaps the organization does several kinds of charity work, each reflected in its own design, in which case the design type is something like "charitable activities". It is very often the case that the stamps of an issue should also have an design type.

Unlike the stamp's issue, design type has no effect on sort order or grouping. It also need not be globally unique, or even unique to an issuer. For instance, if a country issues an annual series of flower stamps, and they are all of generally the same appearance, it makes sense to put each of these into separate issues "flowers 1980", "flowers 1981", but giving each of these the same design type "flowers".

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