Apple offers a service called iCloud Photos that allows you to easily sync the photos you have on your Mac with those you take on your iPhone, iPad, or even another Mac. Using iCloud Photos is optional, and is off by default - you can store your photos on your Mac without doing any syncing to iCloud if you wish.
Only one library can be synced with iCloud at any given time. You must first designate that library as the system photo library in Photos’ preferences, at which point you can enable iCloud Photos, also in Photos’ preferences. When viewing your libraries in PowerPhotos, an “iCloud Library” label will appear below the library’s name.
There are currently not any options provided by Photos to sync a subset of your library with iCloud - it’s all or nothing. PowerPhotos can help with this limitation by allowing you to create separate libraries to store the photos that you don’t want to have synced with iCloud. For example, if you just want a relatively small selection of photos to sync with iCloud, you can set up a library just for that, then store the bulk of your photos in other libraries that are not synced. This helps save storage space used on your iCloud account.
Optimize Mac Storage Photos also provides an option to “Optimize Mac Storage” when using iCloud Photos. When enabled, if your disk starts to run low on free space, Photos will not store the full size original versions of photos on your Mac. It instead leaves the full size copies on iCloud, and only downloads the ones it needs on demand (e.g. when you view or edit a particular photo).
When you have this option enabled, viewing or copying photos or videos with PowerPhotos may mean that Photos needs to download data for some or all of the items being copied. This may cause some operations to take longer to complete, and make your library take up more local disk space.
In addition to iCloud Photos, Photos offers two other way to transfer photos between devices: My Photo Stream and Shared Albums. These two features can be enabled or disabled independently of iCloud Photos, but like iCloud Photos, they can only be enabled for your system photo library. Photos from these sharing options will not appear in PowerPhotos.