0.08, 0.11, 1 and 6 mJy. In addition, the absolute (radial) differences between [[*WISE*]{}]{} source-peaks and “true" astrometric positions anywhere on the sky are no larger than $\sim$ 0.50, 0.26, 0.26, and 1.4$^{\prime\prime}$ in the four [[*WISE*]{}]{} bands, respectively [@cutri11][^4].

$\gamma$-ray blazar candidate definition {#sec:association}
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Based on the $s_b$ and $s_q$ distributions of all [[*WISE*]{}]{} sources in different random regions of the sky, at both high and low Galactic latitudes (Paper III), the critical threshold of the $s$ parameters, used to define the above classes, have been arbitrarily determined on the basis of the following considerations:

-   [class A: [[*WISE*]{}]{} sources with 0.24 $<s_b<$ 1.00 and 0.38 $<s_q<$ 1.00;]{}

-   [class B: [[*WISE*]{}]{} sources with 0.24 $<s_b<$ 1.00 or 0.38 $<s_q<$ 1.00;]{}

-   [class C: [[*WISE*]{}]{} sources with 0.10 $<s_b<$ 0.24 and 0.14 $<s_q<$ 0.38.]{}

All the [[*WISE*]{}]{} sources with $s_b<$0.10 or $s_q<$0.14 are considered [*outliers*]{} of the [[*WGS*]{}]{} and, for this reason, discarded. All the above thresholds are then used to select the [[*WISE*]{}]{} sources that are associated to the UGSs and that can be considered potential $\gamma$-ray blazar candidates.

The above choice of threshold have been adopted for the analysis of the $\gamma$-ray blazar content within the 