In Time Machine, all data is backed up by default. Therefore, rather than define what the source is, you define what the source is not. Once a target location has been defined, the next thing many Time Machine users do is define any data that is not to be kept in the Time Machine backups. This is done with the addexclusion verb. These exclusions are defined using the Options button of the Time Machine System Preference pane as well.
To use the addexclusion verb, simply define a list of items that are not to be backed up as arguments separated by spaces. The tmutil command will then use those items as an array.
tmutil addexclusion /System /Library /Applications /var /etc /Developer /Groups /Incompatible Software /Volumes /bin /cores /usr /tmp /temp /opt /net /home /Shared Items /Network /Groups