What does the idiom lion's share mean?

There are four types of roll up summary fields. They are count, sum, min and max. The purpose of the roll-up summary fields is a step ahead of the usage of fields in a single record like the formula fields do. However, these summary fields must be used on the master object. To do this, you must go to the app setup and then click on create. Next click on objects and click on new field.

Then you can select roll up summary field. You will need to fill out the information next. A true statement regarding roll up summary fields would be to change the value of a rollup summary field so that it can trigger assignment rules to run.