Viktorija asked me to explain the difference between 'collocations' and 'topic vocabulary'.
Collocations are just groups of words that often go together. Many groups of words, from "life expectancy" to "for example", can be called collocations.
Topic vocabulary is the term I use when I'm teaching IELTS writing task 2. It refers to single words, collocations and phrases that relate specifically to the question topic. "Life expectancy" is topic vocabulary for this question, but "for example" is not (because it's a phrase that can be used to organise ideas in any essay).