When there is an available outgroup species, a pair of orthologous genes will have an Ortholuge classification when there is an gene in the outgroup genome that is orthologous to the two orthologs in the comparison species. If this outgroup gene is missing, no Ortholuge evaluation will be available.
Ortholog groups are transitive. That is, they contain all ortholog connections linked to any of the genes in the group (i.e. the orthologs of the ortholog, and so on). Orthologs for a single gene only contain genes directly linked to the gene of interest through an orthologous relationship. Ortholog groups are broader in scope and are better suited for examining the distribution of a gene, while obtaining orthologs for single gene can be limited to specific species and is ideal for more focused queries.