With visibility formulas, you are able to make a custom field appear based on the value or selection of another custom field. In the example below, there is a drop down custom field with two options, yes or no.
With formula custom fields, you can make certain fields appear based on your choice. When you select yes in the above example, a blank number field will appear.
However, this field does not appear when you select no.
If you would like a value to automatically calculated based on one or more custom fields, you will use a value custom field. In the example below, the fields are used to calculate how many tables will be needed for an event based on the number of people attending.
Once the number of people attending value is filled in, the number of tables will be calculated based on that value.