Common data is added to a Work Package because it contains information that is used to fill in fields on each inspection done in that Work Package, or because there is supplementary information to show in the App that does not go into the Forms or Reports (e.g. Health and Safety contacts)
Click on the 'Settings' on your Work Package you will see the Common Data section.
This shows the 'Field Label' on the left and the information for that label on the right.
In edit mode you will see all the text fields from your inspection template that are available to be used as Common Data. If you use those fields and give them a value, then each inspection will have that field filled in when it is created. Some fields should not be used as they are specific to that inspection (e.g. 'Inspection Reference', and are not common across all inspections.
Click on and drag the 'handles' on the entries to set the order you want.
You can add new information, however this will not be used to fill in fields on the inspections as you have added it, and not chosen one of the form fields. e.g. this could be specific information about this job that you would like staff to be aware of.