When you make an entry in your logbook you need to record what vehicle you are driving. If you change vehicles during your work-shift you need to reflect this in your logbook.
To do this you'll need to switch to another state (break or other duty) and then back to drive using the different vehicle record.
Start Your Shift, Select Drive (if thats the work type you are doing) and then select the vehicle you are driving.
When you are ready to switch vehicles click on Other Duties
Wait 1 minute and then click on Drive
This time select the next vehicle you are going to drive
You can see at the top of the dashboard what vehicle you have said you are driving
You can also see in the logbook page that you have selected two different vehicles in your work-time record.
Additionally, the TSL Holder of both of these vehicle records will receive a copy of this work shift when you lock it.
You can see these steps in the images below.
Step 1
Step 2 & 3
Step 4 - 6