| | Extensible Markup Language (XML) 1.0 (Fourth Edition) |
 | | Parameter entity references are recognized anywhere in the DTD (internal and external subsets and external parameter entities), except in literals, processing instructions, comments, and the contents of ignored conditional sections (see 3.4 Conditional Sections). |
 | | However, portions of the contents of the external subset or of these external parameter entities may conditionally be ignored by using the conditional section construct; this is not allowed in the internal subset but is allowed in external parameter entities referenced in the internal subset. |
 | | The external subset and external parameter entities also differ from the internal subset in that in them, parameter-entity references are permitted within markup declarations, not only between markup declarations. |
| www.w3.org /TR/REC-xml (8624 words) |