When stamps are printed, the printing sheet is cut into some number of pieces. For sheet stamps if cut into two panes (the size of a sheet of stamps sold at the post office), then they are labeled Left or Right (L or R). When the printing sheet is cut into four panes for sale at the post office they are labeled UL, UR, LL, LR for upper left, upper right, lower left and lower right. On modern material sometime the position of the pane in the sheet is shown in the margin selvage with a diagram showing the number of panes cut from the printing sheet with the particular pane highlighted grey.
On the pane sold at the post office, the positions are numbered from top left as number 1, the next one to the right is number 2 and so on to the end of the row. Then it returns to the leftmost stamp in the next row lower. That could be number 6, or 11 for example depending then number of stamp images in a row.
Thus Pl 12345 UL pos.1 is the upper left pane of stamps from the sheet printed from and with plate number 12345, upper left most stamp.
Below are panes with the diagram showing the pane position as it appeared in the printed sheet. The diagrams are in the selvage under stamp position 18.
The Polar Bear pane above is the dead center pane of the 15 (3x5) made from the printing sheet.
Below the UPU pane is the lower right of the nine panes (3x3) made from the printing sheet.