It is said that different parts of radish have different tastes.
In general, the top part is sweet and the bottom part is spicy.
For that reason,
The head part is used in stir-fries and salads.
The middle part is fresh and soft, so it can be used for oden and furofuki radish.
The lower part is grated radish
Isothiacyanate (mustard oil) is what makes radishes spicy. Isothicyanate is generated by the reaction between glucosinolate, which is the source of isothiacyanate, and the enzyme myrosinase when radish is grated. In other words, if the radish is not grated, the enzyme will not react and it will not be spicy. (Wasabi has a similar principle)
If you add vinegar to grated daikon radish, the spiciness will disappear.
It is said that the glucosinolate and myrosinase content is high in the lower part, so it is said that the lower part is painful when used for grated radish.
Also, it is commonly believed that the reason why the upper part is sweet is that it undergoes photosynthesis, but so far it has not been fully elucidated.