Yes, 8.00 will delay it by 8ms & -8.00 would play it 8ms earlier. But remember this isn’t moving the actual part it just adjusts when the part plays back relative to its current position. If you want to change the position of the part, adjust its start position on the Info Line.
Regarding Musical vs. Linear Time Base (not Mode which refers to a file setting in the Pool) you can try it in a dummy project and see what happens.
Create Project with 120 tempo and a MIDI Part between bars 4 and 8 on a Track set to Musical TB. Make sure the ruler is set to Bars & Beats.
Change tempo to 80. You’ll see the Part still goes between 4 and 8.
Reset Tempo to 120 and then switch Track to Linear TB.
Change Tempo to 80. Now the Part doesn’t go between bars 4 and 8.
Reset Tempo to 120 and then switch Track to Musical TB. Also change ruler to Seconds and remember where the Part starts & ends in seconds.
Change tempo to 80. You’ll see the Part’s start & end points in seconds have changed.
Reset Tempo to 120 and then switch Track to Linear TB.
Change Tempo to 80. Now the Part’s start & end points are the same as they were at 120.
It’s just a way to deal with the fact that changing tempo moves when bars & beats occur in real time. In 4/4 at a tempo of 120 bar 30 will start at the 1 minute mark. But if you change the tempo to 60, the bar 30 will now start at 2 minutes.
So Musical TB means you are gluing the Part to a timeline measured in bars & beats.
Linear TB means you are gluing the Part to a timeline measured in seconds.