See Note 8 of Table 1-4 in IRS Pub 590-A:
https://www.irs.gov/publications/p590a#en_US_2024_publink1000270086
Paragraph 402A(c)(4) of the tax code is the paragraph that permits taxable rollovers to a designated Roth account. The restriction that it must be an IRR derives from paragraph 402A(c)(4)(B):
(B)Distributions to which paragraph applies
In the case of an applicable retirement plan which includes a qualified Roth contribution program, this paragraph shall apply to a distribution from such plan other than from a designated Roth account which is contributed in a qualified rollover contribution (within the meaning of section 408A(e)) to the designated Roth account maintained under such plan for the benefit of the individual to whom the distribution is made.
