To create a field that only displays when certain criteria is met:
Log into CRM as an Admin User
In the top right, click the User icon, and select Admin
Under the Developer Tools, click the Studio option
Click the module to add this to
Click the Fields option
Click the field to make dependant
Next to the Dependant field, select the check box
The formula option will always present, however if this is a dropdown field you will also have the option to choose a dependent type of Parent Dropdown. For a formula:
Select Edit Formula
Write the formula that meets your required criteria. Hovering over the available formulas will give their syntax.
If you are creating a dropdown field and wish to use a Parent Dropdown dependent type:
Ensure the dropdown list you are using for the main field contains ALL options. (As an example we can use the numbers 1-10)
Under Parent Dropdown, choose the decisive dropdown. (As an example we would use the dropdown Numbers, that contains the choices Odd and Even)
Click Edit Visibility
Here you will drag and drop each number under Odd and Even respectively.
Click Save