A typical xref definition includes objects, such as lines or arcs. It also includes dependent symbols-definitions of blocks, dimensioning styles, layers, linetypes, and text styles. When you attach an xref, AutoCAD LT differentiates the names of these dependent symbols from those in the current drawing by preceding their names with the name of the externally referenced drawing. For example, the dependent symbol that is a layer named STEEL-HIDDEN in an externally referenced drawing called stair.dwg is listed as STAIR|STEEL-HIDDEN.
When you attach an xref, the definitions of its dependent symbols are not added to your drawing permanently. Instead, these definitions are loaded from the referenced file each time you reload it.
A dependent symbol's definition can change. It can even disappear if it is purged from the referenced drawing. Therefore, you cannot reference a dependent symbol directly. For example, you cannot insert a dependent block or make a dependent layer the current layer and begin creating new objects on it.
To avoid the restrictions on dependent symbols, you can use XBIND to bind dependent symbols to your current drawing. Binding makes dependent symbols you select a permanent part of your current drawing.
When dependent symbols are merged into a drawing, you can use them the same way you use the drawing’s own symbol table definitions. After a dependent symbol has been added, AutoCAD LT removes the vertical bar symbol (|) from each dependent symbol name and replaces it with two dollar signs ($$) separated by a number (usually zero): for example, STAIR$0$STEEL. You can change any of these names by using DDRENAME.