[Koha] On basic differences of MARC, MARC21 and UNIMARC

Hello all, "Koha 3.14. Manual (en)" considers "MARC", "MARC21" and "UNIMARC". What principle differences exist between these formats ? My database (more preciesely, isis-catalog, 100 000 records) has been created in accordance with Ukrainian standard "UKRMARC". Which of the indicated formats must be chosen for preferences ? What criteria need to be followed for the choice of marc-format ? Thank you, Gennady