Vasari highlights all elements in the partial chain between the first and last elements that you selected. (Press Tab again to highlight the entire chain. Press it a third time to highlight only the last desired element in the chain.)
If the chain is a closed loop, the elements to be highlighted for partial chain selection are determined by the cursor position when you highlight the last desired element. As shown in the following wall layout, if you position the cursor near the left endpoint of the upper-right wall, the partial chain includes the walls along the top of the layout. If you place the cursor on the right side of that wall, the walls on both sides and along the bottom are included in the partial chain.
Highlighted partial chain with cursor near left endpoint of last wall in chain
Highlighted partial chain with cursor near right endpoint of last wall in chain