What is the difference between on-line storage and off-line storage in the design of storage pond?

The design of storage pond is commonly divided into on-line storage and off-line storage. The on-line storage concept involves the inclusion of storage facilities in series with the pipelines so that overflow at the storage facilities is allowed. One simple application of on-line storage is to enhance a large size of drainage pipes. However, for heavy rainfall situation, the spare capacity of drainage pipes will be rapidly exhausted. On the other hand, off-line storage (e.g. underground storage tank) refers to storage facilities in parallel with the pipeline and the return flow to the main pipeline is only allowed when the outflow pipelines are not surcharged.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.