I have two datatables in two forms, forma and formg. Inside each form there is a p:dataTable, groupsa and groupsg. In each datatable there is a custom column that displays an image(h:graphicImage) called fava and favg. 
When the image is clicked, the images from the other datatable will be updated.
<p:ajax event="click" listener="#{agent.toogleFavorite}"
update="fava, :formg:groupsg:favg" />
Without the colon I get an exception:
javax.faces.FacesException: Cannot find component with identifier "forma:agentsa:fava" referenced from "groupsg:0:favg".
What is the difference between formg:groupsg:favg and :formg:groupsg:favg?
I am using JSF2.0 and PrimeFaces 3.4.