As far as Hong Kong salaries tax is concerned, a much greater tax relief is already given under Section 8(1A)(c) of Inland Revenue Ordinance which is to exempt the income derived from non-Hong Kong services on which foreign income tax has been paid. Because the tax benefit under double taxation relief is generally less than that under Section 8(1A)(c) exemption, DTR is almost useless to taxpayers. 

Even if DTR is to apply --- in such case the mainland-sourced income must be brought into the Hong Kong tax computation --- the tax credit in respect of the mainland tax is restricted to the hypothetical Hong Kong tax payable in respect of the hypothetical mainland-sourced income, i.e. maximum tax credit = H. K. effective tax rate  * mainland's income after mainland tax / (1 - H. K. effective tax rate). In effect, the restriction makes Double Taxation Relief less favourable than Section 8(1A)(c).

Press here for an illustration of computation of tax credit. For more, please visit the IRD's web site and read Departmental Interpretation and Practice Note No. 32.

My comment: A common employee working both in Hong Kong and mainland China should seek relief under Section 8(1A)(c) exemption. If he works only in mainland China, he should seek full exemption of Salaries Tax. Only when Section 8(1A)(c) relief or full exemption is inapplicable (for example the taxpayer holds a director office with a limited company in Hong Kong) should he apply for double taxation relief.