A Scene Block takes on scene specific characteristics when it's the root block in a project (otherwise these fields are ignored).
The Scene Block specific fields largely go with webGL scene specifics - such as fog, canvas color - other fields go with display level animation length, audio and video settings that don't directly map to a webGL scene but live on the same level in the hierarchy of a display.
Fields like Frame Time (frametime) can be used to disable the play button (0) or set the length of the overall animation to repeat. Audio URL (audiourl) and Video URL (videourl) control scene specific media that is played with time elasped in sync with the animation.

Note on HTML 5 Viewer

The screen must always be clicked to start a video or audio file (a user interaction) - otherwise the browser disables the sound/video source. This includes videos used as a texture or for native DOM display.
A scene block is a "macro" that resolves to a block with fields set.

Equirects can quickly fill in the background, using a 360 camera is a common way to acquire custom ones quickly.
"sb:" is a macro that converts to a full path prefix in the cloud. It's used to abbreviate the full path, the image preview will popup and you can right click and view the image and capture the full path if wanted.
Uploading a file stores it in a project specific location using the token as security.