Assasination Types
There are roughly seven ways to (stealth-)assassinate people, not including outright combat situations considering those require your cover to be blown. As a rule, an assassination can only be done while not (fully) detected yet, and the enemy needs to be glowing red.
1. Regular/Same ground assassination (full stealth). This is an assassination of a target at the same level as you. Approach them quietly from the back to perform this. It attracts very little attention.
2. Regular/Same ground assassination (free running). This is also an assassination of a target at the same level as you, except this time you rush over towards them. If done successfully (and it may fail if the target fully notices you beforehand), it draws more attention than usual, but you can compensate this by immediately rushing on to a different place.
3. Double Assassination. This is a contextual assassination on the same ground, where you approach two targets from the back and kill them.
4. Air Assassination. This is an assassination from a higher vantage point. The enemy needs to be sufficiently close below you so they're marked red. This type of assassination draws a lot of attention and the attack takes a little bit longer than usual.
5. Cover Assassination. While in a cover position behind a wall or an object such as a crate, or when hidden inside a haystack, you can assassinate targets that walk very closeby or towards you. If you need to lure an enemy towards your position, whistle by pressing down on the directional pad.
6. Hanging Assassination. When you are hanging from a ledge or a window and an enemy is standing above you, it's possible to pull your target to the streets below, instantly killing them.
7. Rope Zipline Assassination. If you've created a rope with the rope launcher and you're hanging above an enemy, an air assassination is possible.
Eagle Vision
This mode is turned on (only while not Free Running) by pressing L3 / Left Stick. It highlights enemies red, allies green, police blue and objects of interest white (such as hanging barrels, chests, alarm bells, dynamite. You cannot perma-mark objects, but to permanenly mark people, move the camera so that they're aligned in the center of the screen. Upgrade Eagle Vision 2 as soon as possible to make marked people visible/glow through walls. Eagle Vision 3 expands the effective range in which people are marked.
Rope Launcher
You acquire the rope launcher somewhere during sequence three. Press L1 (PS4) / LB (Xbox) when the icon appears at the top of the screen to instantly eject a rope and launch yourself towards the top or near-top of a building. If a grapple point appears straight ahead (when you are, for example, already positioned on a building yourself), eject the rope to travel towards that point faster. When you're ziplinging down a rope, use the left stick to adjust your speed. Hanging above an enemy allows for Air Assassinations. Whoopie.