The Combat Knife (aka Tactical Knife, Hunting Knife, Makeshift Knife, Reinforced Knife, Finely-crafted Knife, or simply Knife) is a Gear item and a Weapon in Rise of the Tomb Raider and Shadow of the Tomb Raider.
Rise of the Tomb Raider
As a gear item it can be used to cut Ropes, e.g. like those that attach Bells to their posts or those that Lara herself has fixed to objects in her surrounding like Ziplines. As a Stealth weapon it can be used to kill Enemies silently, e.g. by sneaking up on them from behind or jumping off buildings when they pass below.
Shadow of the Tomb Raider
Used for cutting rope and attacking enemies.
Press (Triangle) to CUT rope.
Makeshift Knife: Used for cutting rope ans attacking enemies covertly.
Reinforced Knife: Adds the ability to cut through braided ropes.Finely-Crafted Knife: Adds the ability to harvest more animal resources.
When Lara starts out after the plane crash in the Peruvian Jungle, she has lost all her gear. To cut it down from a tree she needs to find a tool. She can find it in form of a piece of one of the propellers of the plane wreck. This is only a very basic tool that needs sharpening first. Lara has to collect three pieces of Salvage and sit down at a Base Camp to do so. The result is the Makeshift Knife.
This Knife is not very sharp and can only be used for the most basic things. It is enough to cut rope and so get Lara some of her gear back, as well as sneaking up on Enemies and Stealthily kill them. But it can not cut through barriers sealed with Braided Ropes. This can only be achieved by upgraded Knifes.
Lara's Combat Knife was called "Tactical Knife" in an earlier Concept Art