1. A communication control method in an image forming system which comprises a plurality of applications and an image forming apparatus and in which at least a print job described in JDF code is transmitted from said applications to said image forming apparatus and a message described in JMF code is exchanged between said applications and said image forming apparatus, said method comprising the steps of: (a) specifying an application based on a description of said print job and said message associated with said print job, transmitted from any of said applications to said image forming apparatus; (b) managing rules of describing said print job for dynamic value including any values assigned per print job, which is added to said print job, per application together with operations associated with said dynamic value; (c) recognizing said dynamic value by analyzing said print job according to said rules managed in said step (b) for said application specified in said step (a) for said print job, and performing said operations associated with said dynamic values managed in said step (b) for said recognized dynamic value; and (d) managing said dynamic value recognized in said step (c), wherein said operations performed in said step (c) comprise: an operation of, when at least one of said print job and said message associated with said print job is received by said image forming apparatus, storing and managing said dynamic values recognized in said step (c) in said step (d); and an operation of, when said message for response or notification to said print job is transmitted to said application specified in said step (a), adding said dynamic values managed in said step (d) to said message to said print job.