Internal text entities are not appropriate when:
the entity is large
the entity needs to be shared with other documents.
An entity may be too large to be conveniently stored in the declaration. It may, for example, consist of several paragraphs of text. XML editing tools are generally ill-equipped for editing the content of internal entities. The entity content may need to be accessed from a number of documents, and it would be both time-consuming and error-prone to redefine it in each one.
An external text entity differs from an internal entity in one important respect. Instead of the entity being held in the entity declaration, it is held elsewhere. Usually, this means that it is held in another data file. This file may ...