Midrange speakers(MT) are speakers that radiate the middle frequency range. At which frequency this frequency range begins and ends is not fixed. This range is rather related to the constellation in which the midrange speaker is used in combination with tweeters and woofers(TT). In general, the lower frequency is between 100 Hz and 400 Hz and the upper between 3 kHz and 10 kHz.
Also, there are no specific statements about the size, power or type of speaker. What matters is the combination and frequency response of the other speakers. The overall frequency response curve of all speakers combined in a speaker cabinet or wall must be such that no frequency ranges are boosted or attenuated.
This means that in a 3-way system, the lower cut-off frequency of the midrange driver should match the upper cut-off frequency of the woofer and the upper cut-off frequency of the midrange driver should match the lower cut-off frequency of the tweeter and is matched via the crossover.