The CCSS Trusted Environment is everything that can impact the security of key material. This includes people, processes, technology, systems, infrastructure, and any third parties involved in key material generation, usage, storage, access, etc.
If a person or systems can approve a transaction, access a key, or influence how key material is handled, they are part of the CCSS Trusted Environment. Same goes for vendors, cloud providers, or external services that play a role.
In practice, defining the CCSS Trusted Environment is about answering:
Who or what could compromise the key material?