There's no ironclad rule saying that each deposit must be accompanied by such-and-such precise kind of documentation.
What they care about is that the totality of your application paints a convincing picture of your economic circumstances, taking into account your situation in general and what is usual in your location for people who can afford going on overseas vacations.
As general rules of thumb:
Document every deposit of a kind where -- in your location -- it is generally assumed that when you receive this kind of money you also get a separate notice to document it. This is in particular true for salaries and other taxable income.
Document and explain other deposits that could look like they make up a significant portion of your economy.
If you run a business and customer/client payments are paid into the same account you use for personal economy, you're sunk. Get a business account for the business transactions, tighten up your bookkeeping, and postpone your visit to the UK until you have separate financial reports that show the business in action.
Small payments that don't count for much in your total economy don't need to be documented separately. What this covers can vary extremely according to local custom in the country you live in. For example, in Denmark it is very common to use bank transfers rather than cash for informal settlements between friends, such as splitting a takeaway bill. (The transfers complete in seconds and the banks don't charge for them). An ECO who sees such transactions on bank statements from a resident of Denmark wouldn't bat an eye. But the same thing in, say, the US where bank transfers are expensive and cumbersome, could well require an explanation.