ADK differentiates between tools and subagents based on the ability to escalate or transfer control (subagents), where as tools are more basic
I think this is a meaningful distinction, because it impacts control flow, regardless what they are called. The lexicon are quite varied vendor-to-vendor
I think in ADK, the most likely place to find them actually used is the Workflow agent interfaces (sequential, parallel, loop). Perhaps looping, where it looks like they suggest you have an agent that determines if the loop is done and escalates with that message to the Looper.
https://google.github.io/adk-docs/agents/workflow-agents/
I haven't gotten there yet, still building out the basics like showing diffs instead of blind writing and supporting rewind in a session
Are there any examples of implementations of this that actually work, and/or are useful? I've seen people write about this, but I haven't seen it anywhere