When you want to modify objects in Ax, you have to buy a development license. They are divided in four.

• Base Package
  This license gives you access for creating/modifying reports/jobs/queries
• MorphX
  This license gives you access tot the Data Dictionary node where tables/Fields/enums/Extended Data Types are maintained. The ability to create forms requires the MorphX security package.
• Web MorphX
  In addition you can modify Web forms.
• X++
  Gives you access to all classes and a development code for the customer (CUS-) layer.

I hope that this gives you a clearer view to the different kind of licenses and witch one you need to have.

(Source: Course AX 2009 – DEV I)