Standing

Upon activation, the block (including air) below the player's feet will become selected. After moving some distance and activating again, the new block below the player's feet will serve as the end point.

Click

Upon activation, the block (including air) the player is looking at within a five block distance will be selected. Moving your crosshair will change the lastly selected block. Activating again will choose that block currently being looked at as the end point.

Non-axis-aligned

This acts as a "free-form" line that does not align to a single, or double, axis. It will create a line from the first selected position to the second selected position.

Axis-aligned, single-axis

This is the default line form in which the line is created by the axis of the largest difference between the two points selected.

Axis-aligned, double-axis

This is similar to the previously mentioned line, but with a caviot. Instead of having just one line, there are essentially two lines formed from the two axes that have the largest difference out of the three total from the two selected points. If you are familiar with Terraria's ruler tool, it would be most similar to that. If you are still confused, refer to the screenshots.

Placeable

Lines are now "placeable" in the world. With this option enabled, the line will stay in place after selecting the second position. Additionally, there will be sliders to offset the line in any of the three axes. Just remember, if you alter the offset, it will STAY until you either reset it or turn off the placeable option. I say this in case you go to place a new line and do not see anything, the offset will still be active when placing a line.