The structure is identical to cultural traits in general, except for a few added fields:

category = string
This is used for grouping in the Add Tradition view, and in the Divergence view.

Layers match the CULTURE_TRADITION_LAYER_PATHS define. Index starts from 0
layers = {
	0 = martial # Use a random icon from the specific subfolder
	# 1 = blah # By leaving out the index, it'll use a random icon from the folder
	3 = letter1.dds # By specifying a file name, that specific file will get used
}

Example:
