What is the difference between HISAT2 index “genome.tran” and “genome”

I try to use HISAT2 in order to map the reads to the genome, while HISAT2 pre-index the genome for users.

I saw that alternative sequences will affect the mapping results, does genome.tran index includes the alternative splicing and alternative sequences compare with genome index?

And what the difference between index genome and genome_tran?

which index file should I use in mapping process?

1. genome

2. genome_tran

3. genome_snp_tran

Thanks for your answering