If you include an XLINE in a Block Definition, when you place an instance of that Block, you will not be able to select the XLINE. This brief Screencast starts out with two XLINEs, a CIRCLE and a square closed POLYLINE. As you can see, the XLINEs can be selected when they are placed directly in the drawing. But after putting all four elements into a Block Definition, you cannot select the resulting BLOCK REFERENCE by left clicking on the nested XLINE or by running a crossing window over the XLINE.
Putting an XLINE in a Block Definition is not something I have ever needed to do, but if you have a use case for it, be aware of this limitation. NOTE: If the Block Definition contains only XLINEs, then you can select the BLOCK REFERENCE by selecting one or more of the nested XLINES. They only become unselectable when there are other object types included in the Block Definition.