For parameter values PENDING means "unknown" and "not (yet) fixed".
General
Realise that "unknown" or "not (yet) fixed" means something completely different than "not available" or "0". A pending value is part of a model but not yet determined. A parameter with the values 0 really has 0 as its value (zero is not nothing...). When a parameter is not part of a model, it will not be available.
Note that Quaestor assigns the value -999999 to parameters with a pending value.
Constraints
For CONSTRAINTs pending means that no sufficient data are available to evaluate either a DETERMINED TRUE or FALSE, implying that the decision to either reject or accept a particular relation in the template may need to be postponed until sufficient data are gathered.
Modeller
The Modeller takes advantage of the PENDING (or DETERMINED) attribute of parameters and CONSTRAINTs in the reasoning process.