Marriage Visa and Fiance(e) Visa are not at all the same.
Fiance(e) Visa is essentially a settlement visa. It is granted for 6 months vailidity. Means that you must get married within the 6 months validity, then once you are married and whilst you are still in UK you apply for FLR (Further Leave to Remain) as spouse.
A Marriage Visa is essentially a special visit visa. It is valid for 6 months BUT the holder MUST return to their home country before expiry. It is intended to allow a marriage (and honeymoon) to take place in UK for whatever reason, but it is not a settlement visa route and it is not a switchable visa. The holder must return home.
Any further requirement to enter the UK again will require a new application.
Hope that clarifies the issue.
