Modifier
Modifier has been around for many years and is the main customization tool within Dynamics GP. This tool uses Visual Basic to write code that changes the behavior of the system. Changes made via Modifier can be basic or complex customizations. For example, a simple customization would be to make a field required on a form or to rearrange the fields that display on the form. A more complex customization would change the way the system functions as opposed to changing fields and whether or not they are required.
A more complex customization can meet defined needs, however great care needs to be exercised to ensure that invasive customizations outside the prescribed guidelines are not being created. This can cause your system to be unsupported via Microsoft and cause great pain and effort when your company wishes to upgrade to a new release of Microsoft Dynamics GP.
Extender
Extender was originally developed by eOne Solutions from Australia. eOne is a long-time Dynamics GP partner and still develops solutions for Dynamics GP. The purpose of Extender is to enable users to make simple customizations without writing code. Many times customizations that companies desire are really more configurations and are simply the need for additional information. For example, a company may want to know additional details about each line on a sales order or record additional details about each customer.
Extender gives you the ability to add an unlimited number of fields to a form in the system. Picklist, text, numeric and currency fields may all be added and a new field may be marked as required. Picklist fields can be especially valuable as they only allow users to specify certain values which provides better data uniformity and integrity.
The final key element to Extender is that any field that is added can be added to a SmartList or SmartList Builder report. Therefore, a company can add fields to the system and then report on those fields and gain valuable insight into their operations.
Summary
The choice between Extender and Modifier is an important one, but usually a fairly easy one. When you wish to change the behavior of the system or create almost entirely new modules then Modifier is needed, but if you just need to track additional information and report on it that Extender will meet your needs.