How is the STT calculated?

Can you please explain to me in simple terms how the STT, or the Security Token Tax, is calculated? I'm a bit confused about the process and I would like to understand the factors that influence its computation. Is it based on the transaction value, the type of asset involved, or both? Also, are there any exemptions or deductions that can be taken into account when calculating the STT? Thank you for your assistance in clarifying this matter.