What is the meaning of clutch fill? See for example this paper.
The paper you referenced defines what the term means.
To ensure precise synchronization [when changing the drive path from one clutch to another one], before clutch engagement, it is necessary to actuate the oncoming clutch to a position where the clutch packs are in contact. This process is called clutch fill...
It sounds like they are talking about engaging the clutch to the point where the friction surfaces are touching and slipping somewhat but without transmitting any significant torque.
The current technology of automatic or semi automatic transmissions uses a pair of wet plate clutches which are better able to tolerate some slip as part of normal gear changes than conventional dry ones. Wet clutches have a smaller coefficient of friction but larger surface area (achieved with stacked plates) and larger pressures so it is practical to take them to a point where the plates are in contact but little or no torque is transmitted without causing excessive wear or heating.
This presetting allows the clutch pressure to be ramped up as desired to accomplish a smooth transfer between the two.