Lines, curves, shapes and text can be rasterized to create pixel layers. This is only a requirement if the appearance of complex vector gradients or effects need to be honored, particularly for print artwork.
The data in pixel layers is ordinarily stored within the Affinity Publisher document.
A pixel layer can be converted to an image layer or a picture frame. By doing this, the resulting layer’s contents become listed in the Resource Manager, where you can select Make Linked to save them to an external file or Replace to overwrite them with data from an external file.