In order to be useful, the documentation should contain sufficient detail to address key needs of several types of stakeholders:
- Business management, who need a high-level understanding of how the data they receive was created;
- Spreadsheet users, who rely on the spreadsheet to aid their productivity and need to understand how to use the spreadsheet; and
- Spreadsheet maintainers, who need to understand the functionality of the spreadsheet at a very detailed level.
As with systems documentation, the most important component of spreadsheet documentation is a description of requirements. Requirements describe functions or tasks the spreadsheet must perform. For spreadsheets that have been in use for some time, the requirements are probably generally understood as the list of things the spreadsheet is used to do; however, a formal description of requirements -- what the spreadsheet must do as opposed to what it can do -- is critical for understanding the spreadsheet. A set of high-level requirements can provide a sufficient overview of the functions performed by the spreadsheet (e.g. "The cash flow projection spreadsheet must calculate projected revenue from sales for the next five years."), while sub-requirements can expand upon high-level requirements to achieve the necessary level of detail (e.g. "The cash flow projection spreadsheet must calculate average annual revenue for each product type based on quarterly sales.").
Documentation should additionally address how these requirements have been implemented in the spreadsheet. This part of the documentation should include all inputs and where they come from, all outputs and where they go, and an overview of the calculations performed by the spreadsheet. It is important to note, in the documentation, interfaces with other groups or departments, which are frequently the source of input data or recipients of output data. If the spreadsheet includes multiple worksheets, it is important to provide an understanding of how the different worksheets interrelate, describing the overall structure of the spreadsheet as well as the inputs, outputs, and calculations of each worksheet as a unit.