In Destiny 2 now, there are 3 weapon slots: Kinetic, Energy, Power. For those 3 weapon slots, there's a multitude of weapons. Each weapon uses one of three ammo types: Primary, Special, Heavy. (Take note that there is no Secondary ammo type. Some people refer to the Special ammo as Secondary, but that's not what it's called.) Primary ammo is received from the White ammo blocks you can pick up. Special ammo is received from the Green ammo blocks you can pick up. Heavy ammo is received from the Purple ammo blocks you can pick up. You can tell what ammo a weapon uses by viewing the information about it in your inventory. When looking In the box with information, you may see an icon at the top right just below the weapon type and to the right of the weapon's power value. If it's a white box with a single bullet in it, then it uses Primary ammo. If it's a green box with two bullets in it, it uses Special ammo. A purple box with three bullets in it means it uses Heavy ammo. It also has the word printed next to the box stating what ammo type it is depicting. Destiny 2 no longer uses a Primary, Secondary, Heavy system. It now uses the Kinetic, Energy, Power system. Because Destiny 1 used the Primary/Secondary/Heavy system, some players may refer to the top slot as Primary, the second slot as Secondary, and the bottom slot as Heavy; they may also refer to certain weapon types as Primary, Secondary, or Heavy (As Notry mentioned), based on the type of ammo they use. With the above info on what might be considered Primary/Secondary, without more specifics on context of why you're asking the question you're asking, it's difficult to give a more directed response specifically for your question.