When does a block,image,layer & text style can not be purged by "Purge command"?

A block definition cannot be purged if
 It is nested inside another block definition.
 It is inserted in the drawing.
An image definition cannot be purged if
 It is nested inside the block definition.
 It is inserted in the drawing.
A layer cannot be purged if
 It is the current layer.
 It contains objects.
A text style cannot be purged if
 It is the current text style.
 It is used by a text in the drawing.