IGCSE English as a Second Language is built for students whose first language is not English and who are still developing it.
What it covers
All four skills: reading, writing, listening and, depending on the syllabus variant, speaking. Tasks are practical: emails, reports, articles, conversations.
The content is closer to daily and academic usage than to literary analysis.
How it differs from First Language
First Language assumes fluency and tests what a fluent user can do with the language. Second Language builds the language itself and is graded against a learner's expectation.
The same effort produces a higher grade in Second Language. That is the point of the course and also the reason the choice matters.
How it is read afterwards
Schools and universities do not treat the two as interchangeable. Some institutions accept First Language at a stated grade as evidence of English proficiency and do not accept Second Language for the same purpose.
Others accept both, and some accept neither and require a separate proficiency exam. The rule is set by each institution and published on its own pages.
Making the decision
Two steps, in this order. First, read the language requirement of the institutions the student is likely to apply to. Second, judge honestly whether the student can meet the demands of First Language.
Doing it in the reverse order is how students end up with a good grade that does not count for what they needed.
Moving up later
A student who takes Second Language and later becomes fluent can sit First Language separately, or take a proficiency exam. Neither is difficult to arrange, and both take time that is easier to spend earlier.