A Drop Rule is a validation rule. Drop is shorthand for Drop From Process.

Drop Rules work in close conjunction with payload state. When a drop rule executes two optional events are available:

1. The data payload in question is pended in K3.  It is not sent further downstream.   

-Persistence set to: True        -The data payload caught by the when statement is saved. 

-Persistence set to: False       -The data payload caught by the when statement is purged.

2. An email alert is generated.

Alert set to: True                         -An email alert is generated about the data payload

Alert Set to: False                       - No alert is generated

Some data sources occasionally populate fields incorrectly.  Let's say that an upstream system sent a field to K3 with no data.   This will cause problems for the Target system.  We can set up a Drop Rule to ensure that empty data is brought to our attention for correction.  

When important_field = null

Then Drop  Persistence = True  Alert = True

This will hold the dropped payload until is manually re-pushed downstream.  See this article.