The ash of any burnt object is not regarded as holy ash.
Bhasma (the holy ash) is the ash from the homa (sacrificial fire) where special wood along with ghee and other herbs is offered as worship of the Lord. Or the deity is worshiped by pouring ash as abhisheka and is then distributed as bhasma.
Bhasma is generally applied on the forehead Some apply it on certain parts of the body like the upper arms, chest etc. Some ascetics rub it all over the body. Many consume a pinch of it each time they receive it.