If genome is a BSgenome (from the package BioStrings), it will transform it into a GRanges with chromosomes and chromosome lengths.If genome is a data.frame with 3 columns, it will transform it into a GRanges.
If genome is a data.frame with 2 columns, it will assume the first is the chromosome, the second is the length of the chromosomes and will add 1 as start.
If genome is a character string uniquely identifying a BSgenome installed in the system (e.g. "hg19", "mm10",... but not "hg"), it will create a genome based on the BSgenome object identified by the character string.
If genome is a GRanges object, it will return it as is.
If genome is non of the above, it will give a warning and try to transform it into a GRanges using toGRanges. This can be helpful if genome is a connection to a file.