The Samsung Galaxy J7 Pro comes with a Dual SM and Dual socket, but the best thing is that it also comes with a dedicated microSD card slot. This means that users won't have to choose between a micro SD card or a secondary SIM card. you can use all three at the same time. The dedicated mSD card slot means that there is a specific slot for you to insert the memory card. You can just put it in and forget about it and enjoy higher storage space.
Some other phones come with a Hybrid slot, which means that you have only two slots. You can use two SIM cards at once but ditch the memory card or use on SIM card and a memory card, you can't use all three. This is really an issue for many users who don't want to give up on the microSD card if somewhere in the future they decide to use the second SIM slot. I have many apps, games, photo and data stored on the microSD card. I don't want to start moving some of them to the phone just because I have a business flight and I need to use a second SIM, one for private calls and one for business. For example, the Samsung Galaxy S8 comes with a hybrid Dual SIM slot with no dedicated memory card slot.
Just for clarification. In the J7 Pro dual-SIM model you have a dedicated SIM card tray and below it the SIM card / memory card tray. So although the secondary tray has two places for card, one is dedicated for the microSD card and the second for a SIM card. When you hear the term "dedicated micro SD card slot" you might think that there is a slot only for the memory card, but this isn't the case here, although in some other setups it is. The only downside of this is that if you need to open the tray to take out the microSD card or put one in, the phone won't be able to operate with the SIM card in that dual-slot tray until you lock it back into the device.
Of course you have the option to either bring a second phone with you and install the second SIM there, but who want to deal with all that hassle? No one. This is why I put this feature in the 'Advantages' section. If you agree, please vote this up. Cheers.