  We present results on heavily underdoped Y_1-xCa_xBa_2Cu_3O_6+y which provide
the evidence that the doping mechanism (cation substitution or oxygen loading)
directly determines whether the corresponding injected mobile holes contribute
to superconductivity or only to the normal metallic properties. We argue that
this hole tagging calls for a subtler description of the correlated bands than
the usual one. We also map in great detail the underdoped superconducting phase
diagram T_c vs. hole doping which shows that the number of mobile holes is not
the critical parameter for the superconductivity.
