To try and explain things better.... The DP3400 has a rotary knob (2) on the tope of the radio with 16 positions/channels to choose from and by pressing the Zone button (6) you get another 16 positions/channels to choose from. The tone when pressing button 6 represents the Zone you are in - a low sounding tone tells you that you have selected Zone 1 and the high sounding tone tells you Zone 2.
If we look at Zone 1 channel 6 this selects the frequency of GB7TD with the slot on the repeater for Talk Group 8 S2 which is the Roaming channel. What this means that as you drive about or walk about and the signal strength drops on the repeater signal you are receiving then the radio automatically roams you onto the next DMR repeater that has been configured into the same Talk Group so you can have a seemless QSo without having to change channels - it's done for you automatically. The roaming Talk Group is great and aimed at mobiles who are say going from York to Leeds to Huddersfield to Manchester and finishing in Liverpool. If there are DMR repeaters at key positions along the route then on the Roaming Talk Group, if their coverage overlaps, you could carry on the same conversation from door-to-door.
If we then look at say Zone 1 channel 4 this selects the frequency of GB7TD with the slot on the repeater for putting your call out to all other DMR Repeaters within the UK which may also have others listening for a QSO or even putting out a call.