Suppose you have some elements (let's say coins) laid out over a table in vertical order. How can I make reference to the coin at the bottom? The lowest coin? The one that is below any other? I just don't know how to say that.
You should use bottommost and topmost. The postfix "most" shows physical position. Use the postfix "most" with adjectives that compare three or more things.
If you have two coins, do not use bottommost and topmost. You'd use bottom coin or top coin. If you have three coins, two are potential bottom coins, since only one is on the top. Because there are more than two possibilities to be the bottom coin, it is appropriate to use bottommost to refer to the coin on the opposite end of the top.
Bottommost: Situated at the very bottom.
Topmost: Situated at the very top.
The one at the bottom is the "bottommost" coin.
If there is a single column, referring to the top and bottom coins in the column is perfectly appropriate.
If there are multiple columns, then topmost and bottommost are (at least theoretically) the less ambiguous way to refer to the coin at the top of the highest column and the bottom of the deepest column respectively, although many people may feel obliged to further clarify with something like "take the topmost or bottommost of all the coins" instead of just "take the topmost or bottommost coin".