Most genomes provided by Ensembl Genomes contain primary gene annotation that has been imported from external sources. This includes the re-use of gene and other feature identifiers assigned by those responsible for this annotation. Suitable identifiers are imported and attached as stable identifiers to the corresponding Ensembl feature. In contrast to Ensembl, stable identifiers are not generated, and mapping of identifiers between different versions of external gene models is not generally provided by Ensembl Genomes.
Note that in some cases, where specific unique identifiers are not provided, they may be generated from appropriate parent identifiers. For instance, unique transcript identifiers may be generated by appending a number to the end of an existing gene identifier.
More information is provided here on the procedure used for assigning identifiers for annotation imported from INSDC.