Variables Tab

Variables are used to handle the state of various workflow related information that doesn't justify a state of it's own. The default CMF workflows use variables to track status history comments, and store the the last transition, who initiated it and when, for example. From the variables tab it's possible to add new variables, and rename and delete existing variables.

The list of existing variables also displays a summary of each variable's description, catalog availability, workflow status, default value or expression and any access guards. You can click through to each variable to configure it's details.

In each variable's property tab you can set the variable's description and a collection of properties the define the variable's behaviour, as follows:

Make available to catalog
Just as it says, it makes this variable available to the catalog for indexing, however it doesn't automatically create an index for it - you have to create one by hand that reflects the content of the variable. Once indexed, you can query the catalog for content that has a particular value in is variable, and update the variable by workflow actions.
Store in workflow status
The workflow status is a mapping that exists in the state_change object that is passed to scripts and available to expressions.
Variable update mode
Select whether the variable is updated on every transition (in which case, you should set a default expression), or whether it should only update if a transition or state sets a value.
Default value
Set the default value to some string.
Default expression
This is a TALES expression (as described in Expressions) and overrides the default value.
See the "Guards" section.

State variable - stores the name of the variable the current state of the content is stored in. CMF uses review_state by default, and will have already created a FieldIndex for it. The state variable is effectively a variable with "Make available to catalog" set, a default value of whatever the initial state is and a default expression that sets to the new state on every transition.