Up here (Canada), "Category" and "Class" have different meanings depending on application. "Single-engine," "multi-engine," "water/sea" and "land," etc., are pilot licensing classes. Hence a "sea" obtained in a single engine seaplane and a "multi-engine" obtained in a multi-engine landplane automatically includes a multi-engine sea, e.g., "single and multi-engine land and sea" (SMELS). I suspect it's the same where you are.
If you're in the US, The FAA says:
Category:
(1) As used with respect to the certification, ratings, privileges, and limitations of airmen, means a broad classification of aircraft. Examples include: airplane; rotorcraft; glider; and lighter-than-air; and
(2) As used with respect to the certification of aircraft, means a grouping of aircraft based upon intended use or operating limitations. Examples include: transport, normal, utility, acrobatic, limited, restricted, and provisional.
Class:
(1) As used with respect to the certification, ratings, privileges, and limitations of airmen, means a classification of aircraft within a category having similar operating characteristics. Examples include: single engine; multi-engine; land; water; gyroplane; helicopter; airship; and free balloon; and
(2) As used with respect to the certification of aircraft, means a broad grouping of aircraft having similar characteristics of propulsion, flight, or landing. Examples include: airplane; rotorcraft; glider; balloon; landplane; and seaplane.
The way I read it, takeoffs and landings in any e.g., "normal category, airplane" is valid for ALL "normal category, airplanes."
But there are caveats. Some aircraft fall into more than one "category," depending on how they are operated. E.g., you can't maintain currency in "utility category, airplanes" by doing the required takeoffs and landings in that same airplane operated in "normal category" - or vice versa.