When a sequence is displayed in the Sequencer, it is represented as a grid. You can zoom in or out of the grid in either the horizontal and vertical direction at any time. See the Scrolling and Zooming topic for more information.
Whenever save your sequence the zoom level is saved too. And when you re-open that sequence, the zoom level will be restored. However, whenever the Sequencer creates a new sequence, it will use some default zoom level settings. If you prefer some particular zoom level, you probably do not want to zoom in or out every time that you create a sequence. So, the Zoom Preferences sub-menu (of the Tools menu) allows you to save your current zoom level settings as defaults, by selecting "Save Zoom Preferences".
Later, when you create a new sequence, it will default to the zoom level settings that you had saved.
If you change the zoom level on a sequence and want to get back to your saved default zoom level settings, the Zoom Preferences sub-menu also provides a way to do this: by selecting "Apply Saved Preferences".
There is also a system default zoom level, and selecting "Apply System Defaults" will apply that to the active sequence.
Zoom Preferences Sub-menu