If you set a store-wide alert threshold then every product variant in your store is checked against that alert level.

You can also set alerts on only specific products or variants each with its own alert threshold Additionally you can set alert threshold on individual products and also enable store-wide threshold for the rest of the products in the store.

Simple check the checkbox under store-wide alerts that says "Enable both store-wide AND single-product alerts with above number as base level" All this is available in the basic plan for $9/month. Here is a 5 minute demo video of Basic plan features.