Branch-site REL will perform a series of LRT tests to identify all lineages in a phylogeny where a proportion of sites evolves with dN/dS > 1, i.e. instances of episodic (dN/dS varies across the tree) diversifying selection at a proportion of sites (dN/dS varies across sites). As such, this model is a generalization of existing branch-site methods.
The details of the algorithm to efficiently compute the phylogenetic likelihood function in this context and further model background can be found in this Molecular Biology and Evolution paper