Emblemata is where the mosaic is set onto a tray of terracotta or marble. This is then taken to the villa and set into the floor. As well as fine detailed mosaics there are also examples where the emblemata is quite simple decorative patterns. It is assumed that these are made off site, there is even a reference to a building in Pompeii having some stacked up, possibly for sale.
Roman mosaic as emblemata. Image copyright, History of Libya (Facebook page) 2017