Generally, the requirements must be gathered however possible, from designs, interviews and meetings, etc. Documented assumptions help to paint a picture of the requirements as they are currently defined. Documenting and publishing assumptions provides an opportunity to highlight problem areas early in the life cycle of the project.
Assumptions are factors underlying the estimate. They must be clearly and fully documented. These assumptions will help support the estimates and also serve to generate discussion about the requirements. Assumptions will be especially useful for poorly defined requirements. An unsubstantiated estimate can easily be ignored, while one supported by assumptions is more likely to gain either criticism or agreement of the assumptions, both of which can produce a useful discussion.
The assumptions can serve as a useful review mechanism to gather more definition and clarification of the requirements. Use assumptions to highlight potentially under-defined areas of the system by indicating where the size or an HLR is unexpected relative to other requirements or where the percentage of identified ILRs is very low. Assumptions will bring focus to frequently omitted requirements (performance, volume handling, etc.) and gain the needed discussion to adjust the estimated size and also encourage further definition.
When requirements are very poorly defined, the analysis and generation of assumptions is critical. In fact in these cases, the process of generating the estimate can be as useful as the estimate itself.