It means all assets that use Special Depreciation Allowance.
However, your comment "in the past, bonus depreciation could be applied to some assets, but need not be applied to all" is not really correct. You can't randomly choose bonus depreciation for some assets and not for others. If you elected out of bonus depreciation, that applies to all assets in that "class". So if you elect out of a 7 year property, all 7 year property can't use bonus depreciation for that year.
https://www.irs.gov/publications/p946#en_US_2017_publink1000299556
