Workflow

Workflow doesn't have an interface, in simple terms workflow are the series of chained events that gives you extra advantage of manipulating the responses/output/actions.

https://docs.llamaindex.ai/en/stable/module_guides/workflow/

However another product of llamaindex: Llama-deploy which uses workflow has a SDK that allows you to interact with deployed services directly.
It allows you to use workflow within it, helping you to achieve a high level of customisation.

https://docs.llamaindex.ai/en/stable/module_guides/llama_deploy/#llama-deploy

Ok and both Llama-Deploy and LlamaIndex Workflow are open source?

Yep fully open-source :LlamaIndex:

And LlamaDeploy can use the latest version of LlamaIndex Workflow product? In terms of the workflows it deploys and manages?

These two products are in version sync ? As far a latest LlamaDeploy can use latest LlamaIndex Workflow ?

Or are there some version recommendations or compatibility considerations?

the llama-deploy package should work with the latest llama-index-core package yes

If I have multiple requests in a RAG deployed in llama-deploy, will it serve the users concurently/async or not?

It will yes (its async/concurrent by design)