How much is the 300 baht tourism fee in Thailand?

Excuse me, could you please clarify something for me? I'm trying to understand the cost of the 300 baht tourism fee in Thailand. Is this a flat rate that applies to all visitors entering the country, regardless of their nationality or the length of their stay? Also, does this fee include any additional services or is it simply a mandatory charge? I'm just trying to get a clear picture of what I can expect to pay as a tourist visiting Thailand. Thank you in advance for your help.