Resources can be either different formats of the same data (such as XLSX and CSV) or different releases of the same data (such as March, April, and May needs assessments). Always put the resource with the most-recent or most-important information first, because the HDX system will by default use the first resource to create visualisations such as Quick Charts or geographic preview (this default can be overridden in the dataset edit page).
If you have data that is substantially different, like a different type of assessment or data about a different province, we recommend creating a separate dataset.