The channels with a + beside them mean they are 40MHz wide channels instead of the standard 20MHz wide Wifi Channels. So Channel 1+ is going to cover all of the channel space for both channels 1 and 6. This will get you faster speeds (the full 300mbps for the OM2P-HS) but if you are in a dense wifi environment like an apartment building you will run into more interference and you will be causing more interference for your neighbors. Because of this we only recommend you use wide channels on more isolated networks.
Because the 5GHz band is less congested, all channels are wide and will be designated with a +.