WhenDidI has a number of measurements pre-installed. However, you are not limited to the supplied measurements, you can add you own quiet easily.
Measurements are labels in WhenDidI, for scalar values. So they do not need to be restricted to real world measurement types. They also used to provide measurement information when the event details are displayed, such as the events list, or the chart axis information.
To add a new measurement, go to the main screen and click on the more tab, which is the last tab in WhenDidI. Select the measurement icon, which will bring up the current list of measurements defined.