Data are the core puzzle element. A data element is a collection of files etc that somehow makes up a group of data that shares certain characteristics and are handled in a similar way.
You can have as many data elements that you like. E.g. you can have a data element representing a collection of photos, one that represent a collection of documents, or you can join these two together. The level of abstracion is what makes sense to you. If you apply different processes for one part of the data, and not for another part this might be the determining factor if these two should be divided.
Examples of data:
A collection of images
Collection of images and text
A collection of survey data in a database