The third person narrative is primarily filtered through Catherine, hence the limited part of the omniscience. We get most of the story through her eyes and we are also privy to some of Catherine's inner thoughts and feelings, which is an experience we lack with most of the novel's other characters, like Isabella.
The third person narrator has limited omniscience in the sense that we don't have full access to all the characters. But we are also hindered by Catherine's limitations and blind spots – to a point at least. Since Northanger Abbey is a comedic satire, it relies on irony. Irony is a pretty big term that can mean lots of things. So let's see what kind of irony the narrator uses. Basically, irony is all about opposites. If you are being ironic, or sarcastic, you say the opposite of what you actually mean. The narrator uses irony to be funny, like Juno. Here's an example of irony at work:
imbecility in females is a great enhancement of their personal charms. (14.28)
Jane Austen doesn't really mean that. If she did, she probably wouldn't have sat down to write a bunch of really intelligent novels. There are also lots of instances of dramatic irony in the book. Dramatic irony is where the narrator lets the readers know what's up, while the characters remain clueless. For example, we get lots of hints that James and Isabella like each other before Catherine figures it out. So the narrator often indirectly clues the audience in to events and actions that elude, or escape, Catherine.
Austen is a tricky narrator, and the book's narrative is complicated by the fact that it is a satire. The narrator often tells us information about characters in order to make fun of them. Some of the "inner thoughts" we receive from characters are not so much actual thoughts as they are Austen's witty, interpretive spin on them. Hence, the limited omniscience here. Omniscience might be stretching it a bit at times, too. Austen's narrative technique is often something akin to third person psychological profiling for the purpose of mockery.
The narrator also ditches the third person as well. Often, the narrator directly addresses the reader Ferris Bueller-style and uses the first person "I." So Northanger Abbey is mainly an example of third person limited omniscient narration, in that we primarily stick with Catherine and get the story from her perspective. But the narrator often departs from this model, which makes classification a little tricky, albeit interesting.