First things first

Your goal: Coherence

The design of a tag strategy, like any other standardization, requires coherence.The name of your keys, the value of your tags, your APIs. Everything must be done intelligently to make sense. If you already have some kind of convention, you will have to migrate it to your new standard. It is difficult for users to deal with two rival standards.

Tagging is achieve through automation

Tagging requires rigor, despite the fact that it is a repetitive and boring task. Fortunately for us computers excel in this area. Tagging should not be apply by a human, but through automation. This means that your infrastructure provisioning processes need to be automated.

Multi Cloud Tagging

If you are on a multi-cloud strategy, your providers may have different tag constrains. You'll have to establish a compatibility between your providers. Some cloud don't accept capital letters tag keys, others don't accept dots... Some limitation are listed in Resources/Cloud Tag Limitation.

Even if they are often n separate teams managing n clouds, don't silo your teams in charge of your standardization. Otherwise you risk getting duplicate tags between your suppliers, it's silly, inconsistent, and your internal infra teams won't adhere to a wobbly standard.

Involve the Finance department

The more you get into the cloud, the more money you'll spend. Sooner or later you will discover the role of the Comptroller. And they will rightly want to know how and for whom these resources are allocated.

As an IT person, you don't have the vision of how the company is financially structured. Your tags should allow you to break down the cost of the infrastructure in your organization. So ask the finance department, the way they run the business to design the best possible categorization in order to facilitate the work of the FinOps team.