The GSR parameter on Nexus FF cells: https://github.com/YosysHQ/yosys/blob/master/techlibs/nexus/cells_sim.v#L95 which controls whether or not the FF is reset by the global set/reset signal is shared within a pair of SLICEs (total of 4 FFs).

It looks like the current constraints schema only allows cell type -> tag mapping; this sort of thing also necessitates cell parameter -> tag mapping.

This could probably be similar to how different cell parameters can map to different cell->bel pin mappings.