Invoking a removal operation on the retrieval buffer explicitly cancels any requested retrievals and resets the buffer state. In canonical ACT-R a retrieval error cannot be reset except by a successful retrieval, which can often be a hassle to deal with.
Retrieval module does permit indexed (i.e. immediate) retrievals of chunks you already have references to. However, this must be explicitly enabled with the EnableIndexedRetrievals parameter.
Retrievals can also be performed asynchronously if the declarative module is asynchronous.
Not implemented yet.