Hmm, I’m guessing it’s just an error (or she read it wrong), because if you were on the 10-year route, you would know it already, because your previous visa would have been granted under the 10-year route.
The 5-year route is the standard route and almost everyone is on it.
People are only put on the 10-year route if their application falls for refusal and UKVI determines that instead of refusing the visa outright, they can be granted a visa outside of the rules under Article 8 (right to family life in the U.K.).
So, if you are not already on the 10-year route, you can’t be put on it now unless you don’t meet the requirements for this application - which won’t have been determined yet.
Also, if you were applying under the 10-year route now, you would be applying for FLR(FP), not FLR(M)... what does your application communication say on it? Is it FLR(M) or FLR(FP)?
If it is as she confirmed - FLR(M) - then the application should be for the 5-year FLR(M) route and not the 10-year FLR(FP) route.
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