Preconditions allow you to execute a post function under certain circumstances only; essentially they are conditions for the post function component of your rule. This allows you to create complex behavior on your post functions, enabling Jira to work harder for you.
JSU's preconditions are a part of the post function section in Jira's workflow configuration. In the example below, a precondition is added to the Clear Field Value post function.
Choose your precondition
In the post function configuration page, select Add Precondition then select Add for the precondition that you want to evaluate.