How am I supposed to know I shouldn't be using "n" or "N" to decode the network data?

That's a tough question to answer in full. Essentially, you have to know what format the data you are trying to decode is in.

For example,

• Fields 1 & 2 are both 4-bits, hence the template you have extracts them both as a single 8.bit number, which is presumably broken down further later in the code.
• Field 3 is is an 8-bit number, and extracted as such.
• Field 4 is a 16-bit number, hence they use 'n' to extract it.
• Field 5 Ditto.
• Fields 6 & 7 are 3 & 13 bits respectively. But as #7 crosses a byte boundary, they extract the two as a single 16-bit number (the third 'n'), and then (presumably) break that down furtehr later in the code.
• etc. ...

Without knowing what the format of the data you have is, there is no way to know what templates are applicable.