The preceding four paragraphs talk about a lot of different kinds of entities; and another classification, internal vs. external, will show up in another section or two.
One might conclude that this profusion of entity categories is a symptom of a rather ad-hoc design, and this may well be true; entities are used for a lot of different purposes, and their use involves three distinct syntaxes.
To summarize, there are the following ways to categorize entities:
This makes a total of eight possible combinations; but it turns out that neither internal/unparsed nor parameter/unparsed can happen; so in fact the number of different entity categories is five:
This is why it's fine to refer simply to an "unparsed entity" or to an "internal parameter entity". It is important to note, though, that unparsed entities are general entities in that they share a name-space with other non-parameter entities.
Copyright © 1998, Tim Bray. All rights reserved.