This patch sets selection groups for all shared structures in the generic parent templates of structures (as is already done for catafalques).
Clicking behaviour in 0 A.D. is as follows (unchanged):
* single click: select the entity in question
* double click: select all entities of the same selection group within view that are owned by this player (e.g. grouping b/a/e versions of the same unit)
* triple click: select all entities of exactly the same file within view that are owned by this player (i.e. the same civ, rank, structure it is trained at, wall length, etc.)
Without this patch this means that if you've built e.g. farm fields of your own and captured some built by other civs, double (or triple) clicking on one will select only those that were built by the same civ, even though they're basically all equivalent.
With this patch double clicking will select all your fields within view, regardless who originally built them (and triple clicking will select only those of the same civ). The same applies to centres, houses, etc.
This will probably make playing 0 A.D. somewhat more convenient for both new and competitive players.