Darpa Definition: Extended solids are polymorphs/phases of simple molecules that are currently formed under ultrahigh pressure conditions where strong intermolecular bonding and tight crystal packing can be induced, leading to dramatic changes in physical, mechanical, and functional properties.
Darpa talks about extented high pressure phases. By this they apparently mean phases that can be induced by high pressure although high pressure is not needed to maintain them. This is not a very good use of the word, extended, which has been used to refer to the case where atomic or molecular bonding is reasonably uniform.. I assume they actually mean materials bound by an electronic lattice. The strength and QED properties of the electronic lattice often confers exceptional properties on the materials.