File, Group, and Component Attributes: http://www.keil.com/support/man/docs/uv4/uv4_ca_filegrp_att.htm
The little red "No Entry" symbol is supposed to indicate, "Items excluded from the target and linkage"
If I manually exclude an item from just one Target, the symbol is still displayed in other Targets from which the item is not excluded!
This is misleading and, thus, unhelpful!