This is just an intro to general scene possibilities in SAC, not so much a step-by-step How-To.
A general description: A scene stores, for later recall, some set of settings taken at a particular point in time. While a scene recall within SAC can be very granular, internally a Scene stores everything about the state of SAC's input, return and output channels when it is created or updated. This allows you to create or update a scene, then later modify the actual settings and channels affected. This is particularly handy since you may discover after creating a scene that you're recalling either too much or too little information and you can adjust the recall without having to recreate the settings you originally wanted. As a side note, scenes require the creation of a session file since they are stored within a Scenes sub-folder in the directory where you create your session.
Some terminology: Scenes can be created, imported, updated and deleted. The properties of a scene can also be updated. Creation, importing, updating and deleting all impact the channel settings underlying a scene. Updating the properties only impacts the selection of settings that are recalled, no changes are made to the settings underlying a scene.
Channel selection: Channel selections stored within a scene are made on a per-mixer basis. Do not forget this, they do not cascade across mixers. They behave just like channel selections behave. Select a channel on the FOH mixer and flip to a monitor mixer. Note that same channel is not selected on the monitor mixer. The behavior here is consistent. While this may be a pain if you want just certain inputs selected across multiple mixers, it does afford a depth of flexibility that is otherwise unavailable.
There are several things you can do to help speed the process of channel selection. The first is to store a channel selection on one mixer as a group. Right-click on a mixer window header, hold down Shift and click on a group. Give it a name if you want. Now flip to another mixer, hold down Ctrl, Right-click on a mixer window header and click on a group. The Ctrl modifier will add the selection on the current mixer to the current selection across all mixers. While it would be nice to store the selection across multiple mixers as a group, you can't. You can however store the selection within a scene. Click the New button in the Scenes window and give it a name. Perhaps call it "Select" or prefix it with "Select" so you know what the scene is for. If any of the input, return or output settings are checked, hit the respective Clear All button, make sure the Mixer Chan Order is not selected and you can Clear All the mixers as well. The only thing you want checked is the Select Partial Chans option. Viola! Your complicated channel selection is now stored for easy recall.
Another handy tip is that you can adjust the partial channel selection while you have the Scene Properties window open. Click on a mixer view, add or remove channels, click back on the Scene Properties window and click the Update button at top left. You can double-check the channel drop down just to the right of the button to verify your selection was updated. Speaking of that drop-down box. Its only purpose seems to be displaying the channels you have selected. You can't select a channel from the list and make changes in the recall selections for just that channel.
Updating a scene: Make your adjustments within SAC, select a Scene in the list (careful not to double-click the scene) and click the Updt button at the top of the Scenes window. Make sure you select the proper scene first as the entire SAC state is overwritten onto the currently selected Scene. You can seriously hose yourself if you forget that but it's an easy lesson to learn the hard way. If you accidentally forget, hopefully you remember in time to hit Cancel on the confirmation dialog. Once you progress to the Scene Properties window the settings are stored. At that point there is no way to exclaim "Ooops", or something more severe, and cancel the update.
Updating a Scene's properties: Like updating a scene, make sure you select your desired scene first. Whatever is the currently highlighted scene is what gets modified. Right-click the scene list and you'll get the standard message about whether you want to update the Scene Properties or not. Say yes or no depending on what you want and you can change up the Scene property selections when the window opens. Unlike updating a scene, updating the properties is non-destructive. Check and un-check boxes all day long without any worries that you're modifying any of the underlying settings.
Scene Properties selections: From here, the rest is pretty simple really. Select the channels you want to recall with a scene and select as much or as little as you want to recall in the Scene Properties window. The interaction of the selections is also fairly simple. The big thing is to pay attention to how the selections interact. You have 3 types of channels; Inputs, Returns and Outputs. The selections are granular enough to capture all the major sections of each channel type. To the right of the channel data there's a big list of all the mixers. Select the ones you want to affect. What is recalled by a scene is the intersection of the partial channel selection, the channel data selections and the mixer selections. Do note that you can select all the partial channels you want on a mixer. However, if the mixer itself is not selected for recall in the Scene Properties window, nothing on that mixer is recalled.