Kegg module M00021 is descriptive of Cysteine biosynthesis, serine => cysteine module, which is part of the Amino acid metabolism category and Cysteine and methionine metabolism subcategory.

In General the list of Ko entries of M00021 Kegg module is reported. Each Ko entry is clickable and redirects the user to the specifc Kegg Ortholog page with further info about its identification within the genomes in association with the Orthogroups classification.
In Profile + homologs the distribution of the Kegg entries of Cysteine biosynthesis, serine => cysteine module is displayed. Given a Kegg entry and a genome, a red box indicates that one or more loci have been annotated with, while a green box indicates that no locus/loci has(have) been annotated with it, but it/they is/are part of an Orthologous group in which other members have been annotated with that Kegg entry, so it is likely that this copy(ies) also performs this Kegg function.
If all the Kegg entries required by the model to be funcitonal are present, the letter 'C'' is reported, otherwise 'I' singals an incomplet module.
Note that a Kegg module may accept alternative Ko or set of Kegg orthologs to be functional. This means that some missing Ko do not impact the module completeness.

Check the composition of M00021 Kegg module