Any values that are reused across your tasks and hooks can be defined as app variables. Those variables will be available in every hook.
Variables might be added in two places
App Settings - here you are also able to change variable scope and properties
Hook Step - any variable added here will have a initial scope limited to this hook only
Additionally you can also change variable value in Variables tab in the task dashboard. This is particularly useful when you want to change value of variable before each task run.
Variables can be of following types:
One thing to keep in mind is that when creating Order, Customer, Product, Product Variant or Collection variable you will see label like the one below (Example T-Shirt):
This is just a label for the field. Actual value is Shopify's product id!