For parents or adult children that wish to know are they identical twins (monozygotic) or fraternal twins (dizygotic).
To confirm if twins came from a single egg or two eggs at conception to know if they share the same DNA or, like siblings, share half their DNA. Many twins or parents of twins wish to know for curiosity, however, there are medical implications that can be important including:
Blood or organ donation
Disease risk
Testing takes ~5-10 working days and is for peace of mind and is not legally admissible in court.