To apply custom validation to your field you need create a list first and then add rule to the list. Apply the created custom validation list to your form filed will trigger rule checking when do form submitting. Only the filed match all rules in the list then the form can be submitted ( or send out notification ).
Create a custom validation list
Add rule to the list
After list saved you may choose the rule you want to check from the dropdown.
Different rule may have different settings and the settings will display after you choose a rule.
There's description about rule settings and they are easy to understand.
For custom validation the validation message associated to the rule when you add it to list.
Multiple rules in a list
You may choose multiple rules to add in a list to do the special checking for your filed.
The following screen has three rules in the list and all rules combine asks user's input for the field start from No.
Apply the list to form field
All rules in the list will be checked when a list apply to form field.
Validation message in front