The Linkage is the shared soul-dimension, common to all Microns. What some call their sparks are simply terminals to this dimension; unlike Cybertronian sparks, their life-essence never resides within. Nor does it need to return to the Linkage at the moment of death; it is born there,and never leaves the Linkage.
The Linkage has been compared to the Allspark, but the two are separated by the Cerebus singularity,so that Cybertronian sparks may not travel to the Linkage; nor may Micron souls visit the Allspark.
Microns can draw power from an unknown source, through the Linkage. Other machines or mechanoids can also do so, while "powerlinked" to a slave Micron, but this practice is abhorrent and unlawful.
Like the Allspark, the Linkage provides the Microns with a form of afterlife (though they say this term is inadequate, since their souls are always there), and a handy multi-purpose expletive.