We have found some interesting things, but we are stuck after finding 1 set. How do we continue?
There are 26*7 words here. NATO is one of them. Each category is a very specific list of 26 words. Try googling several words you think are from the category together, especially uncommon words or specific phrases. Also, surround each word in quotes so Google knows you're looking for instances of that exact word/phrase, not similar words. You should be able to identify the complete categories then. (Note that not all words start with the letter they represent)
We have found all the sets, now what?
Try replacing all the words in the grid with their corresponding letters from A-Z, and then look for a hidden word/phrase that will give you a hint on what to do next.
I found a word, but I can't do it!
This numberlink is unique. There are 2 ways to start this logic puzzle. The first is to think about the long paths first: The left side has a wide open space, but in reality quite a few paths have to go around, in particular the Tom Thumb set has to go around the Rogue Monster set, which has to go around the Matilda set. The other is to think about the short paths first: The NATO alphabet and LAPD code words are both short paths, and work from there.
How do I extract a 5 letter word?
Once you are done with the numberlink, looking at the paths from A to Z for each alphabet, you should notice exactly 2 words from the same set appear along the path. Extract their respective letters and identify an ordering for a clue phrase to your final answer.