Generally, the search-match (and hence the figure-of-merit calculation) is carried out for all entries in the reference pattern database. However, if any additional information about the sample (like chemical composition, density etc.) is available, it is strongly recommended to use this information by restricting the search-match procedure to entries fulfilling these criteria. The main advantage of this limitation (further called "restraining") is the reduction of the number of candidate entries in the candidate list which makes the final selection of the matching entries by the user a lot easier.
The additional information you have about your sample (e.g. the chemical composition) is entered on the so-called "restraints tab" on the upper right-hand side of the screen. Each time you update your restraints (e.g. by selecting an element that must be present), Match! will update your candidate list accordingly right away, so that you can instantly watch the effect of your additional information (the so-called "perpetual restrainting").
Note: It is of course perfectly possible to use Match!'s restrainting facilities without any diffraction data, i.e. to use Match! as a comfortable retrieval software for the reference pattern database.