Good definition of a Pull production system.
A pull production system is one that explicitly limits the amount of work in process that can be in the system. […] a push production system is one that has no explicit limit on the amount of work in process that can be in the system. (Hopp and Spearman “To Pull or Not to Pull”)
A True pull has an upper limit on work in progress, WIP
The Kanban system dictates a fixed upper limit on the work in progress. You cannot have more material than what is allowed by the kanban card ( or signal). The limit is explicitly defined by the kanban card ( or signal )
Hopp, Wallace J., and Mark L. Spearman. “To Pull or Not to Pull: What Is the Question?” Manufacturing & Service Operations Management 6, No. 2 (April 1, 2004): 133–48. doi:10.1287/msom.1030.0028.