* Country: Credit systems differ drastically internationally. The US system, for example, is very different from those in the UK, Canada, or Australia.
* State/Province: Even within a country, requirements can vary by state or province.
* School/Institution: Individual schools and colleges often have their own specific credit requirements. Some may have more rigorous requirements than others.
* Type of Degree: A bachelor's degree typically requires far more credits than an associate's degree.
The 30-credit number you mention might be specific to a particular high school, college, or program. To understand why *that specific program* requires 30 credits, you'd need to consult its curriculum or catalog. Generally, credit requirements reflect the amount of coursework deemed necessary to achieve a certain level of competency or knowledge in a given field of study.