In an Alfresco Activiti process definition each property in the workflow task model corresponds to a variable available in your workflow.
For example the Alfresco supplied BPM task model defines the property bpm:assignee. To reference this property in your process definition you would specify the string bpm_assignee. Note that the colon character is replaced by an underscore.
Variables in workflows exist at two levels; the process execution level and the task level. If you set the value of a variable in a task, the new value is not available at the process level. If you want to use a variable across tasks, or between a task and conditional flow, you need to copy the variable to the process execution level. Process level variables are available to tasks and sequence flows.