Many commands operate on a contiguous range of selected steps. Steps are selected via right button dragging within the track steps area: right-click on the initial step, and while keeping the right mouse button pressed, move the pointer until it's over the final step, and then release the right mouse button. To clear an existing step selection, right-click within the position ruler at the top of the steps pane, or press the Esc key.
A step selection may be entirely within one track, or may span multiple tracks, in other words it can be one-dimensional or two-dimensional. A multi-track step selection is created using a diagonal motion: right-click one of its corners, and then while keeping the right mouse button pressed, move the pointer to the diagonally opposite corner, and then release the right mouse button.
Some commands only accept a rectangular step selection. A multi-track step selection is rectangular if all of its rows contain the same number of steps. This isn't necessarily the case if the tracks involved have different lengths, particularly if you select near the end of such tracks. If the step selection isn't rectangular, and you try to execute a command that requires it to be rectangular, you'll get the error message "Invalid step selection." For example, the standard editing commands only accept a rectangular step selection.
Some commands can operate on either a track selection or a step selection. For such commands, if a step selection exists, it has priority over the track selection, meaning the track selection is ignored in that case. This can potentially cause confusion, particularly if the steps area is scrolled so that the step selection is no longer visible.