AutoCAD LT treats an xref as a type of block definition with some important differences. When you insert a drawing as a block reference, it is stored in the drawing database and is not updated if the original drawing changes. When you attach a drawing as an xref, you link that referenced drawing to the current drawing; any changes to the referenced drawing are displayed in the referencing drawing when it is opened. A drawing can be attached as an xref to multiple drawings at the same time. Conversely, multiple drawings can be attached as external references to a single drawing.
You can use Content Explorer to attach xrefs as well as insert blocks into a drawing. Use Content Explorer for simple attachments, previewing external references and their descriptions, and quick placement by dragging-and-dropping. For more information, see Using Content Explorer.
You can control the visibility, color, and linetype of an xref's layers and make these changes temporary or permanent. If the VISRETAIN system variable is set to 0, these changes apply only to the current drawing session. They are discarded when you end the drawing session or when you reload or detach the xref.
AutoCAD LT uses demand loading to increase performance with large external references that have been clipped using AutoCAD, or that have many objects on frozen layers. With demand loading, AutoCAD LT loads into memory only the data from the reference drawing that is necessary to regenerate the current drawing. In other words, referenced material is read in "on demand." Demand loading works in conjunction with the system variables INDEXCTL, XLOADCTL, and XLOADPATH.