It’s working age HB Reg 10(3A), but it only makes sense if you follow up the external references.
To be habitually resident, the claimant must have the right to reside, but Reg 10(3A) lists certain EEA rights of residence that do not count – even though you have the right to reside under European law it is not enough to satisfy the HB habitual residence test. Included among these non-pukka rights of residence are the general three months right to reside for any reason you like and the right to go looking for work in another member state.
But in JSA, the right to go looking for work in another member state does count as a pukka right to reside and so, provided you are actually habitually resident, you can claim JSA(ib) while looking for work. That in turn passports you onto HB: people who get IS/JSA(ib)/PC automatically pass the HB habitual residence test.
The above applies to jobseekers after three months as well – it’s just that after three months have gone by, the initial no-strings-any-reason-you-like right of residence lapses and job seeking is all you have left. Again, being a work seeker on its own won’t pass the HB habitual residence test but it will satisfy the JSA one (subject to actually being habitually resident) and that in turn will passport you onto HB.