The Plate tab is Riffyn is a place to document how materials in your experiment are related across each plate transfer you perform. Each step in Riffyn is a material transformation and runs represent each material transformation on the step so each well in a multi-well plate is represented as a resource on a run.
Tracking wells in this manner allows you to track each resource as it moves through your process. This is especially valuable when you have complex well-to-well relationships that are more complicated than referring to plate as a bulk set of data points.
The Plate tab can be found in the bottom panel of measure mode, more details on the plate transfer interface. The Plate tab allows you to set specific transfer types and recall past transfers you or a collaborator have used.
Information on the theory and background of plate transfers can be found here.