The Restrictions sub-tab is relevant for fitting. For each fitable parameter there is a box for enabling the restriction which is defined by the Lower bound and the Upper bound value. The purpose of parameter restrictions is to ensure that the resulting parameters remain within a reasonable physiologic range. Without restrictions, there is a higher chance to end up with a meaningless solution. Note that these bounds are also used for the sliders on the Increment pane.
The Search value is used as an initial step-size for iterative fitting. It is recommended to set this value to about the same size as the parameter value itself, as it is automatically decreased if needed. This approach is not successful if the value is very small from the beginning.