A tracker is not the same thing as the asset it's attached to. When you move a tracker from a decommissioned trailer to a new one, PortaPro preserves the history against the tracker while attributing new positions to the new asset.
Assign or Reassign
Under the hood PortaPro closes the current assignment record with an end-time and opens a new one. The historic assignments are how questions like "where was unit #1019 in March?" stay answerable even after the tracker moved.
One Active Assignment Per Device
A tracker can only be attached to one asset at a time. Assigning it somewhere new automatically detaches it from the old one.
Unassigning
Choose Nothing (unassign) in the dialog. Position data keeps flowing against the tracker itself, but the map won't attribute it to any asset until you reassign.