If you are selling your old computer or upgrading, you must deactivate the license first. Otherwise, you will "lose" one of your two activations.
For Perpetual Licenses (LR5/LR6):
Help > Deactivate.For CC Subscriptions:
An LR Device License Key is a software license string used to activate and authorize a specific device or instance of the LR device/platform (commonly provided by vendors that require per-device activation). It ties licensing terms (features, usage limits, support) to a single hardware device or virtual instance. lr device license key
The LR Key framework relies on asymmetric cryptography (Public Key Infrastructure - PKI) to ensure that keys cannot be forged or tampered with. LR Device License Key — Quick Guide How