I am trying to figure out how to have fields update based on a field input from another module.

One example is I have a field in my deals called "Job Number" that auto-generates when a job is created.  I want that input/number to show up in a field on my Contacts also called "Job Number" when the associated deal to said contact is created.

There is lots of other examples I need to do as well but that is the most critical and I'm sure if someone can help point me in the right direction with that, I can use the basis of it for my other cross-module field mapping I needs. 

In other CRM's I've used, you can do this pretty easy with an out of the box solution similar to conversion mapping in Zoho.  However, it can be performed in any direction (ie backwards in this case) to any module on any specified fields that should contain identical values in multiple modules but I am not finding such in Zoho.  Is a custom function the only way to do this? and if so what would the script be?