As to be expected not all the portals you find are going to be friendly controlled. Sometimes it will be controlled by the opposed faction and it is in these instances that it is important to know how to properly attack and capture an enemy controlled portal.
You can gain action points or experience based on the damage you do to a portal if you destroy a resonator you get 60% of the experience gained for adding that resonator. The same goes for destroying links and fields between portals.
Destroy an enemy controlled Link/Field by attacking the Resonators that are deployed on the Portals that anchor the Link. For example, you can fire an XMP to damage a Resonator and reduce its XM health. When all eight resonators have their XM health reduced to the Critical level, then the Link will fail. You can destroy a link without completely destroying the portal and causing it to go uncontrolled.
Note: The quick action menu items (via long press on the map) usually fires the highest level item in your inventory. If you go to your ITEMS view you can select a specific XMP.
Tip: The damage of an XMP is directly correlated to how close you are standing to the resonator that you are trying to attack. You can see how close you are to resonators by looking at your map in game.
Trick: Higher level bursters do more damage to lower level resonators but may be harder to come by. You will do a minimum of 1% damage to any resonator within range of your XMP no matter of the level or burster or resonator attacking. It is best to tailor your XMPs to the level of resonator you are trying to attack.