This mark means a card is from the first print run of a card set, and that adds value and scarcity
Custom Fit for Standard Trading Cards Boros sleeves are standard size for trading cards, including Magic: The Gathering, Pokmon and Yu-Gi-Oh
Atarka Red 4 Aloe Alchemist 4 Heartfire Hero 4 Monastery Swiftspear 4 Monstrous Rage 4 Play with Fire 4 Kumano Faces Kakkazan 4 Atarka's Command 4 Burning-Tree Emissary 4 Copperline Gorge 4 Stomping Ground 4 Karplusan Forest 4 Reckless Bushwhacker 3 Cunning Coyote 2 Ramunap Ruins 2 Den of the Bugbear 2 Cragcrown Pathway 1 Sokenzan, Crucible of Defiance 1 Mountain 1 Audacity 3 Magebane Lizard 2 Rampaging Ferocidon 2 Rending Volley 1 Shapers' Sanctuary 1 Grafdigger's Cage 1 Scorching Shot 1 End the Festivities 1 Bonecrusher Giant 1 Kari Zev's Expertise 1 Pick Your Poison 1 Tears of Valakut Atarka Red is an aggressive deck that aims to flood the board with hasty creatures and subsequently boost all attackers with Atarka's Command or Reckless Bushwacker

In the Archer's Charge/Aim skillset, it's not really useful in general but about any of the charge levels beyond +5 are squarely this, and going beyond +3 is pushing it, as they simply take too long to charge up and the enemy will easily be able to move out of the way before it goes off unless you immobilized the enemy beforehand
I am sure that some people will disagree with me here, but for those of you who have been around long enough, I know you respect this elf as much as I do