Historical relationships are one of the tools and techniques for the PMI process to determine budget. These relationships can be used as part of a parametric estimating or analogous estimating. In other words, the historical data can be used as parameters or in a similar situation to make an estimate. For instance, if you know how much the resources used to open a new branch store cost for a variety of store sizes and the cost of some key factors such as retail rental rates, you can use data from previous branch openings and interpolate.
This is described in section 126.96.36.199 of the PMBOK, 5th edition. This section also points out that historical relationships work best when the historical information is accurate, the parameters are quantifiable, and the models are scalable.