Hello Tkanakar, welcome to the Seven Forums.
If you use two displays in duplicated mode the maximum resolution on both displays is the smaller of your two displays. For instance, if your laptop is capable to show 1360*768 and your secondary display 1920*1080, both displays can only use 1360*768.
In extended desktop mode you can set resolution independently on both displays, in this example case you could have 1360*768 on laptop display and 1920*1080 on secondary display.
Kari