Need help understanding Homestead exemption and if the ISD has any influence on it

HCAD on its website says

School taxes — all homeowners. If you qualify for the homestead exemption, you will receive at least a $40,000 homestead exemption on the value of your home for school district taxes.

County taxes — all homeowners. Harris County currently provides a 20% optional homestead exemption to all homeowners. This means, for example, that if your home is valued at $100,000, the exemption will reduce its taxable value for Harris County taxes by $20,000 to $80,000.

Its the "optional" part in County taxes which I am not clear about, what is optional here and who decides it? Does an ISD have any influence on it. We are looking to get a house under Waller ISD and their 2023 homestead exemption release (https://waller-cad.org/wp-content/uploads/2023/09/hs-2023.pdf) mentions a $3000 exemption under county taxes for flood control, etc. but the 20% reduction in appraised value is under a separate line and again as optional. So I am confused again about the optional part, is the 20% reduction in appraised value available to everyone or not?