The term' initial assessment' isn't a set, special procedure for INZ. It just means that your file has come to the top of the CO's work pile (as they do everything in order of the date it arrived with them), and she has had her
first look at the case, to see what is there. While doing this, she has immediately seen there are issues with the partnership section on which she needs more information
from you, so she has asked. The evidence in other parts of your file (e.g. education, work) will be verified with other people, not you, so how quickly things go will depend in part on how soon your qualification provider and previous employers respond to what your CO will have asked them. When your answer, and theirs, gets back to INZ, these replies will be added to your file, which, meantime, will have gone back to the bottom of the CO's work pile, while she moves on with other people's cases. Gradually, your file will move up the pile again, and when it is next in turn to have the CO's attention, she will see whether she now has all she needs, or if there are more questions for anyone. When a file comes to the top, and ALL replies are there, and satisfactory, that is when a CO can move to the next stage, which is usually to contact you to arrange an interview.