Files shared can be made public, available to all, or kept private with access only allowed to individuals or groups or members of a specified chat channel.

For more information on selective sharing, see Share Groups

To create a Group for file sharing permissions, from the Library view, click the Groups button, then from the menu that appears, click "+Add". After creating a group, the menu allows you to move it up or down the list of groups, to rename it, or remove it.

To create your Share Library, simply click the +Add button. Then on the popup dialog, select the folder/directory or drive you want to share and enable the file types you intend on sharing. These types currently include:
Audio, Video, Images, Text, Other.

When a folder/drive is added to the share library, it is enabled by default for the types of files enabled. These permissions can be changed later. Also, and critical, you can control share permissions on EACH file in the folder/drive added. The entire folder/drive can be disabled and then selective files enabled. Or conversely, the entire folder/drive can be enabled and then selective files within can be individually disabled. Whatever works best for you.

And as stated above, sharing a folder/drive/file can be disabled publicly, but enabled for a specific individual or group.