If they have shields,the same element type. Guns like hard light can switch between elements.

There is also a weapons perk called “Adaptive Munitions “ which well adapts your weapon to do more damage to any elemental shield. It’s not as effective as matching the shield type outright, but you’ll break any shield type, especially during these damned match type shield modifiers. Idk if you were around for it, but the “chain of command” lmg ritual gun has it. Energy primary weapons can have that perk too, as well as even some kinetic ones. The calus dungeon pulse rifle for instance.

So that symbol refers to the “element” of the weapon, of which there are various shields that have the same color. Matching that element does bonus damage and makes the shield explode. Matching is crucial when the modifier “Match Game” is active due to the shield taking little damage from anything but the matching element. This idea also works with your subclass choice. Guns such as Dead Messenger, Borealis and Hardlight are capable of switching to accommodate any elements, while Arbalest has a unique function of being able to efficiently break any elemental shield in the game. Sometimes, matching your guns with your subclass can be beneficial due to synergy that you can create, either through perks such as Incandescent, Voltshot, Repulsor Brace, or Headstone. There’s also a mod called “Font of Might” where if you pick up an elemental well that matches your chosen subclass, it gives you 10 seconds of 25% increased damage with any weapon you have on that matches your subclass element. In regards to what race has what shields, one of the guys on here answered it most of the way, only neglecting the shields that Taken have. More often than not, when you go into a higher level activity, it will tell you in the modifier readout what shields you will you will find, which is helpful of you don’t have a clue who has what shields. I think that about covers it.

everyone feeding the troll

Solar for Cabal, void for legionnaires and arc for Colossus. Arc for Fallen Captains, solar for shanks, void for Servitors. Arc for Hive Knights, Void for Acolytes and Solar for Wizards. For Taken, Malfeasance or Wish-ender if you are inclined.

rocket launcher is normally good against most enemies

Hard Light, good primary against everything, has all the symbol thingies built in except Stasis, but there are currently no stasis shield to break, so no worries. Make sure the Omolon fluid on the gun matches the color of the enemy glow (selectable by holding the reload button, cycles from purple, to blue, to orange), and fire away until you hear a satisfying "pop". Shotgun or fusion rifle to finish majors and linear fusion rifle to finish the ultras and bosses.

What you’re talking about are elemental weapons, and their respective damage types. There are five main kinds of damage types in the game for non-exotic weapons: Kinetic: pure mechanical damage, and dies the most damage against unshielded targets. But it does less damage against shielded targets. Then there are three Light-based types of elemental damage (Arc—blue spark, Void—purple swirl, Solar—orange flame) and one Darkness-based damage (Stasis—blue crystal) for a total of four. All elemental weapons do more damage against shielded targets compared to kinetic weapons. But less damage to unshielded targets compared to kinetic weapons. You’ll do even more damage if you make the damage type to the enemy shield type. Arc: Use to break the shields of Arc-shielded enemies for bonus damage on nearby enemies (Fallen Captains, Hive Knights, Vex Harpies). Or when Arc-burn is a modifier. Void: Use to break shield of/ bonus damage to Void-shielded enemies (The Taken, Vex Minotaurs, Cabal Psions, some Cabal Centurions or Cabal vehicles, Fallen Servitors) or when Void-burn is a modifier. Solar: Use on Solar shielded enemies (Hive Wizards, most Cabal Centurions, Fallen Shanks) or when Solar burn is a modifier. Stasis: [i]There are no stasis shields.[/i] Stasis will do bonus damage to mid- matched shields. But will slow enemies, and synergize with Stasis class-abilities if your weapon has perks like Headstone. With the Light subclass overhaul, elemental weapons have additional abilities if you have the right builds. Arc: will Jolt enemies if your weapon has the right perks (Volt) and create Ionic Traces to give you ability energy if you have the right build in an Arc subclass. Void weapons: will do bonus damage (I forget the name) if you have the right build in a void subclass. Solar: (my favorite) will do massive AoE damage by spreading Scorch and explosive damage on kills if you have the right weapon perk (Incandescent) and will synergize with the same subclass ability if you are rocking a Solar subclass and/or right build. But you have to have the right weapons. Though you’d think this would apply to Solar weapons with firefly and dragonfly perks…..but it doesn’t. Finally there is a sixth kind of “elemental damage” in the game but is only available on the exotic gear: “poison/decay”. This does “damage-over-time” to either single enemies or groups of enemies. Right now it’s only available on Weapons of Sorrow (Exotics based on Hive-based Darkness) like Thorn and Osteostriga, and the Warlock exotic gloves Necrotic Grips. Necrotic Grips synergize with Weapons of Sorrow so weapon kills will spread the poison to nearby enemies just like melee kills with the gloves will do. Hope this helps.