It's pretty clear from the context that this is what it's referring to.

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Note added at 15 hrs (2022-10-21 07:42:19 GMT)
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A word of explanation as to why answers like "factoring" and "invoice discounting" are, in my opinion, best avoided here.

Asker said "'Mobilisation' sounds distinctly odd". Given that it's one of those abstract French nouns that crops up in all kinds of places, one might ask what exactly it's doing here. Think of it as "leveraging" or "putting to work", so that "Mobilisation du poste clients" might be understood as something like "leveraging [your] accounts receivable" or "putting [your] accounts receivable to work" – in other words, using your accounts receivable as a way to generate a financial benefit for your company.

Here in the UK, there are two main ways to do this: factoring and invoice discounting. While the two are similar, they are by no means the same (look them up if you want to understand the differences). The umbrella term that encompasses both these variants – and, by extension, other variants used in other countries – is "receivables financing". And that, in my opinion, is what "mobilisation du poste clients" is referring to here.

In short, neither "invoice discounting" nor "factoring" is the best way to go here because both are both too specific.