1

Celtx projects are called "containers" because they can contain any mix of scripts of varying sorts, storyboards, sketches, documents, notes, media files, and so forth literally thousands!

2

Celtx uses fewer resources (memory, disk space, computing power) because features, such as PDF formatting especially, are done out in the "cloud" (that is, on servers accessed through the Internet, a free service provided by the developers of Celtx)