For a variety of reasons it is not typical that a process runs so smoothly that material enters the system, goes through each process unit once, and your product is on the first try. For this reason there are a few "special" types of streams that should be mentioned:
A recycle stream is one where a portion of the outlet of a process unit is combined with fresh feed and sent into the same unit again.
There are several reasons one might employ a recycle stream:
A bypass stream is one where a portion of the inlet to a process unit is split from the feed and instead of entering the process is combined with the outlet from that process.
This practice is far less common than recycle, but may be used if your ultimate goal is a material with properties "in-between" the untreated reactant and the process outlet product.
A purge stream is one where a portion of a recycle stream is removed from the system in order to avoid accumulation of undesired material in a recycled system.
This is common with multi-phase systems where only 1 phase is either removed or recycled (i.e., if one recycles catalyst pellets, but adds "make-up" fresh catalyst a purge will be needed to discard some "spent" catalyst).
Perform mass balances (degree-of-freedom plus calculations) for systems with recycle, bypass, and purge streams