Semi-formal tuxedos is a grouping of costume codes, representing the sort of clothing worn to events with a level of procedure between informal and formal tuxedos. In history it was used as a morning or evening wear zoot suits dress code, but will now be seen about exclusively in the background of evening wear. In the daytime (by six o'clock) it would have been a black tail-less man coat with formal (stripy or checkered) trousers (this mixture is called zoot suits stroller in America). For evening dress in, it is usually identical with black tie (AmE: tuxedo suits), but the explanation and portrayal of zoot suits dress codes in diverse social settings and continents now vary.
Terms like 'Tuxedos informal' or 'Zoot semi-formal' are used to explain a categorization of tuxedos dress, so would not often be straight given on an invite; instead, the way for the guest will usually be 'lounge suit' or else 'black tie' (AmE: 'wedding suits') where suitable, which remove some of the ambiguity.