External variables are created to access within conditional actions across various forms. 

For example, a user may want to only collect specific lab tests based on the patient's gender or demography. The user would need an external variable for the demography data, so a condition can be built on the Labs form to hide or disable the necessary fields.

On the study tab, select External Variables.

Current external variables can be viewed, edited, or deleted (by swiping). 

To see which fields an external variable is being used, tap the link at the very bottom.

To add a new variable, tap the "New" link in the middle and complete the form.

Name - Name your variable something that makes sense. When building conditional actions, this will help you identify what that variable is for.

Form - Where the variable currently exists.

Field - Based on the form selected, which field do you wish to use as the variable.

Interval - Many forms are collected across multiple intervals/visits as determined in the scheduled visits setup. This is why an interval must be specified - so the system knows which data point to look at whenever that variable is being used in a conditional action.