A set of documents that are designed for a person who is computer literate, probably the person who will be responsible for maintaining and and improving the computer system once it is operating. The documentation will consist of things like lists of variable names that are being/have been used. This is essential if more than one programmer is working on the project because they will need to use the same variables. Alse necessary in the finished documentation will be the program code, fully annotated to explain to the reader the purpose of each section/line of code. Data structures must be explained and justified. Testing procedures must be stated and the results given so that they can be repeated to ensure that the same results are obtained in the future.
Think in terms of a technician being called in because the checkouts and the local supermarket have stopped accepting barcodes printed at the meat counter. The technician cannot solve the problem unless they know how the system is supposed to operate. Similarly imagine that the store is undergoing an exoansion and is to sell 50,000 different products instead of 30,000. This will have a major effect on the way that the system stores and searches the data. The technician will need to know how the structures work before being able to change them. Imagine also that the central office changes the operating system used on their computing systems - what will be the effect on the store computers and the communications between the store and the central office? These are normal problems encountered in the use of computers by businesses, all of which require specialist personnel to solve them and also require those personnel to have specialist knowledge both about systems in general but also about the system in particular and that is where the need for a technical manual comes in.
This should accompany the system and give the user basic instructions about how to use the system. It will contain instructions relating to the users' view of the system rather than any indications about the underlying software. Contents will generally include how to wire up and look after the hardware, input and output formats and a list of error messages with advice as to what to do, general maintenance tasks and where to get help if necessary. The user guide will be written in language appropriate to the users' ability rather than in computer jargon.