The OP is likely speaking of "Constraint Logic Programming(CLP)".

The set of CLP languages is generally implemented as a superset or add-on package(s) to a Prolog implementation. For example, SICStus Prolog and SWI-Prolog have CLP modules, e.g.:

https://sicstus.sics.se/sicstus/docs/3.7.1/html/sicstus_32.h...

http://www.swi-prolog.org/pldoc/man?section=clp

and "Constraint Handling Rules(CHR):

http://www.swi-prolog.org/pldoc/man?section=chr

To the best of my knowledge the modules are written in Prolog. A look at those links will give you an idea of how CLP is used. There are modules available for CLP(X) where X is one of:

B = boolean,

Z = integers,

Q = rational numbers,

R = real(floating point) numbers,

FD = finite domains (see "CLP(FD) Constraint Logic Programming over Finite Domains" http://www.pathwayslms.com/swipltuts/clpfd/clpfd.html )

etc.

One can understand how constraining the domain of interest (reducing the search space) to say, the integers, might make search more efficient.