What kind of data type to use for numbering in the Harmonized System?
"The harmonized system uses a numbering" - they are not numbers, they are codes.  They just happen to use digits instead of Latin letters for the code values.

As you have to retain leading zeros anything numeric will not suit.  You have to go with textual types.  Suppose the codes were

A01        LIVE ANIMALS
A01B03     swine, live
A01B03C10  Purebred breeding animals


I suspect you would have no concerns about using text?  Just think of them this way, but with the A, B and C removed.  Or think of your alphabet as consisting of 36 characters, a to z and 0 to 9.