Pin

Re1

The rook moves to e1 and attacks the knight on e4, directly in front of the black king on e8 on the same file. The target is the king itself, which always counts, so no value comparison is needed. Every move the knight could otherwise make leaves the e-file, so the pin is real. That is exactly an absolute pin.

Re1+

Pin differs from Skewer by which piece the line constrains and exposes. In a Pin, the less valuable front piece cannot move without exposing the more valuable rear piece. In a Skewer, the more valuable front piece must move and expose the rear target. In the counter-example shown here, no piece stands between rook and king, so the position is an open-line check that is neither motif.

The knight is off the file, so no piece of the king's colour stands between the rook and the king on the same line. The file is open, and Re1 is now a check and not a pin. What changes is whether a piece stands in between at all. No comparison of values is involved.