Describe functions of data flow diagram
After you have roughly designed data flow diagram, you could write a description of each function and you could describe the function in any form such as, decision tables, pseudo code, mathematical equations, or some other appropriate form. You need not know implementation of function, but what it does. The description can either procedural or declarative.
A declarative description specifies relationship between input and output values, and relationship among the output values.
A procedural description specifies a function by giving an algorithm to compute that algorithm. The purpose of algorithm is only to specify what function does.