Here’s some documentation on Default Actions: https://help.hcltechsw.com/bigfix/9.5/platform/Platform/Console/c_actions.html
Basically, if the default action is set, then the action that it points to will be the one that gets executed or set when performing various Bigfix activities.
For example, if you were to right click a fixlet, or set of fixlets, in the Bigfix console, you would see an option to “Take Default Action”. This makes it easier to deploy a large set of fixlets without having to individually select the action from each fixlet.
Another example would be if you right clicked a set of fixlets in the console and added them to a baseline. In the baseline creation dialog, the actions for each fixlet would automatically be set to the default without having to specify the action you want for each fixlet.
The SQL Server fixlets don’t have a default action because there are two actions that perform differently depending on the SQL Server environment… .so it’s unclear what should be the “default”.