Ready-to-wear is clothe create for the general market and sold in standard sizes. For wedding or evening dresses most of the time, a seamstress make alteration on the dress to be sure that it fit well.
The Semi-Bespoke, it is what we are thinking about when we talk about on-measure clothes. The client has to choose a model and the tailor takes its measures and creates the exact same model. There is no creation, you have to choose the dress that you like on a book. The tailor will take all your measurements and create the exact copy of the dress that you choose.