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An action on the merits means an action related to patentablity. It excludes formalities like errors in payments or requests to have drawings improved.
From a very quick search of the MPEP (Manual of Patent Examining Procedure), found under
609 Information Disclosure Statement [R-10.2019]
Likewise, an information disclosure statement will be considered if it is filed later than three months after the application filing date but before the mailing date of a first Office action on the merits. An action on the merits means an action which treats the patentability of the claims in an application, as opposed to only formal or procedural requirements. An action on the merits would, for example, contain a rejection or indication of allowability of a claim or claims rather than just a restriction requirement