If your offer letter states that you are required to have In-sessional academic English support, it means that you are a new Phd candidate, and this could be for a number of reasons.
1. You submitted IELTS or TOEFL (or other) test evidence
Normally this means that the assessment of the test has identified a skills area or a number of skills areas that fall slightly below our normal minimum unconditional English language requirement, e.g. scoring less than 6.0 or 6.5 in IELTS in one or more of the four skills sub-scores. So if the requirement for speaking is 6.5 you may have scored 6.0. This would indicate a shortfall in speaking and you would be asked to take the Seminar Speaking Skills course.
In most cases a student will be required to take an in-sessional English course that focuses on the weaker skills area, e.g. Academic Essay Writing (writing), Seminar Speaking and Presentation Skills (speaking), Reading and Note-taking (reading), Listening and notetaking skills (listening) or Grammar Improvement (writing and speaking).
2. A significant amount of time may have passed since you completed your previous studies.
In this case, students are normally asked to follow a 6 week 'Academic Essay Writing' course in order to regain a greater academic focus to their English and learn the conventions of essay writing in UK HEI.
3. Your previous study may have been predominantly mathematical or scientific, and therefore may have involved little essay-based work.
In this case, students normally follow a 6 week 'Academic Essay Writing' course.
4. You used English as the medium of study at university but it did not take place in a country where English is the first language.
In this case, students may be asked to follow an 'Academic Essay Writing' course in order to familiarise themselves with the expectations of UK academic discourse in Higher Education.
5. You used English as the medium of study at university but references accompanying your application mention areas of your language which need further development.
In this case students are required to follow a course or courses which would best suit the nature of the language weakness identified in the references, e.g. Academic Essay Writing, Seminar Discussion and Presentation Skills, Grammar Improvement.