How To Create Reusable & Modular Test Scripts

Test Fragment:
Test Fragment element is a special controller which can be added directly under JMeter test plan like Thread Group. But It does nothing except holding other elements inside!! It gets executed only when it is referenced by a Module/Include controller from other Thread Groups. It acts like a library of reusable of scripts.
Module Controller:
Module Controller in JMeter is useful in referencing any of the logic controller in the JMeter Test Plan.
Parameterized Controller:
Parameterized Controllers contains the ‘User Defined Variables’ section, where you can specify your parameters. Put the credentials of the first user in the first parameterized controller and the second user credentials in the second parameterized controller.
Include Controller:
As a Module controller is used to call a logic controller in the Test Plan, Include Controller is used to reference an existing .jmx file itself.