Imagine that you wanted to tell someone, "We have 50 pounds per hour of water flowing into our tank from another tank." How would you write that down mathematically? How would you write it so that you would be able to remember what you had 50 pounds/hr of?
We advocate a standardized way of accomplishing this. In all of our pages here (and in all of our course materials) we always use an F to describe a flowrate of something. Then we use subscripts to tell us more information, like which chemical in the stream we are interested in, or even which stream we are looking at. When we say stream, we actually mean "pipe with stuff in it". As an example, we could say "We have 50 pounds/hr of water flowing into our tank in stream number 1. We could also write this as:
The first subscript tells us that we are interested in stream 1. The second subscript tells us that we are interested in only the water in that stream (if there is more than just water there).
We go through a distillation example problem on a related page. If you'd like to see how we can write down information from a given problem statement, click here.