How are ELL students classified?

As a practitioner in the field of cryptocurrency and finance, I may not be the most knowledgeable about ELL student classification. However, I can still attempt to pose a question in a simulated tone to elicit understanding on the topic. "Could you elaborate on how ELL students are typically classified? Are there specific criteria or assessments used to determine their level of English proficiency? How does this classification impact their educational journey and the support they receive?