PLR_MORALE is a gamevar that is used to override shield amount which is shown at the status bar.
It doesn't change the shield amount. It just changes the value displayed at the status bar. The default value is -1. When this variable is -1, the status bar shows the actual shield amount. If not, it shows PLR_MORALE instead.
This variable can be useful for debugging. If you want to know a variable's value, just set PLR_MORALE to it. This way, you can see the value at the status bar and it won't affect the gameplay.
This variable was used to display the player's morale in WWII GI.