Taxpayers can opt for their total income from employment or other sources to be assessed on an aggregate basis as under salaries tax?

I freelance in HK. I found this paragraph in a letter from Hong Kong’s Inland Revenue Department. They didn’t embold anything, I embolded all the jargon I don’t understand. Can someone translate all this to simple English? What the heck does "aggregate basis" mean? "same manner as under salaries tax"? Thank you.

Personal assessment allows taxpayers to opt for their total income
from employment, or other sources, to be assessed on an aggregate
basis in the same manner as under salaries tax. This enables a
taxpayer to take the following advantages:

(a) to completely enjoy personal allowances

(b) to deduct approved charitable donations of not more than 35% of
total income

(c) to deduct interest expenses on money borrowed for producing
taxable property rental income

(d) and to offset tax losses sustained under one source of income,
against other sources of income.