Merlin creates new activities having the default work specified in the project settings:

Should you assign a resource to this activity, Merlin will automatically define its work as 100% of the work defined in the activity.

So when to use fractions in work (or duration) and when not to?

The answer is simple: It depends on what you want to do.

Do you plan sub-activities or assignments knowing that they require a fix amount of work (or duration) to be completed? Just enter the value in time units.

Do you plan sub-activities or assignments and want them to adjust their work (or duration) accordingly to changes on the higher level activity? Then enter the value as a percentage in fractions.

To toggle between time units and percentages, use the ‘Fraction’ check box.