Building for the game is building items to shore up your weaknesses against the enemy team comp, or to drill holes in their defenses, or to double-down on an overall strategy like split-pushing or sieging, or to focus on a particular playstyle for your champ depending on your needs.
Compare Shiv and RFC. Both are good for poking, though Shiv does more damage, especially once you build more crit. Shiv also helps quite a bit with waveclear. RFC, on the other hand, offers a range advantage, as well as the ability to poke down turrets, particularly in a siege situation.
As a marksman, you'll often be trying to take down towers. If you're planning to build Shiv or RFC, make your decision based on how the game seems to be playing out. Are you grouping with your team to pressure one lane at a time? RFC, so you can chip away at the tower effectively even if the enemy team is defending it. Are you frequently pushing lanes by yourself, taking towers while your team holds their attention elsewhere? Take Shiv, so you can blow up minion waves quickly and sprint down that lane or rejoin your team more quickly.
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