When a project is launched through a chain, the system applies environment variables from the calling project. They are applied differently depending on whether the called project is called as an Inline chain or as a conditional chain (Pass Chain or Fail Chain).
Called project server environment
Calling project's variables, in a set, with BF_ variable names changed to BF_CALLER.
Called project server environment (applied a second time in case it was modified by the caller's variables)
Called project environment
Step environments (if specified) as they are executed