Taking a partial derivative with respect to a model

"[...] These variables give a measure of correspondence between signal and model relative to all other models, . A mechanism of concept formation and learning, an internal dynamics of the modeling fields (MF) is defined as follows,

[...]

Parameter determines the iteration step and speed of convergence of the MF system; can be interpreted as a number of signals associated with or coming from a concept object . As already mentioned, in the MF internal dynamics, similarity measures are adapted so that their fuzziness is matched to the model uncertainty. [...]"

http://www.leonid-perlovsky.com/4%20-%20Mehler.pdf (page 368)

What does it mean to take a partial derivative with respect to a model i.e. ?

Thanks in advance,

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