JMeter

Variables don't have to vary - they can be defined once, and if left alone, will not change value. So you can use them as short-hand for expressions that appear frequently in a test plan. Or for items which are constant during a run, but which may vary between runs. For example, the name of a host, or the number of threads in a thread group.

When deciding how to structure a Test Plan, make a note of which items are constant for the run, but which may change between runs. Decide on some variable names for these - perhaps use a naming convention such as prefixing them with C_ or K_ or using uppercase only to distinguish them from variables that need to change during the test. Also consider which items need to be local to a thread - for example counters or values extracted with the Regular Expression Post-Processor. You may wish to use a different naming convention for these.