In the metro, your t+ ticket allows you to travel on the entire network.
In the RER, your t+ ticket allows you to travel only within Paris. Any travel outside of Paris requires the purchase of a point-to-point ticket (Île-de-France ticket), the price of which is calculated on a station-to-station basis.
For example, to go from Paris to La Défense - Grande Arche by RER A, you must have a valid point-to-point ticket for this station.
It is however possible to reach this destination by metro (M1), using a t+ ticket. T+ tickets allow you to make metro/RER and RER/RER transfers within Paris. In this case, you must again validate the t+ ticket you used when you first entered the network.