In addition to the various play ranges available on the play menu, the Sequence Editor also supports another way of playing a sequence, using the space bar on the keyboard.
When the space bar is pressed, the current sequence will start playing, starting at the start of the current selection and ending at the end of the sequence (this is equivalent to "From Selection" play mode). Pressing space again will stop play, and the selection will be changed to the spot where play stopped. So pressing it a third time will start the sequence again, approximately where you had stopped it. This can be used to effectively pause and unpause play.
However, the space bar may have a different meaning, allowing play in another way:
During play, pressing the keyboard's down arrow will mark the current time as the start of a "freeform play range". Later pressing the up arrow will mark the current time as the end of the freeform play range. The freeform play range is displayed with a slightly darker grey background color for its cells:
A sequence with a freeform play range starting at about 37 seconds
If the space bar is used to start play while a freeform play range has been selected, then instead of playing in "From Selection" mode (as described above), it will play from the start of the freeform play range to the end of it.
Using "shift-space" instead of "space" to start play will first get rid of any freeform play range that may be selected. The freeform play range can also be removed (without starting play) by selecting "Clear Freeform Play Range" from the right-click context menu.
The freeform play range can also be selected (while not playing) by clicking and dragging the mouse on the time scale, or cleared by clicking (and not dragging).
Note that using the space bar to play does not affect the currently selected play mode from the play menu, so playing in any other way (such as using the Play button on the Standard toolbar) will still use the last-selected play mode.