The Heads in FHB: Flow and Head Boundary package pane is on the MODFLOW Features tab of the Object Properties dialog box.
The data that can be specified for the specified heads in the FHB package are the Time and Head.
Time is the starting time for the specified flow rate or head. The times need not correspond to the times when stress periods or time steps start or end. The FHB package will interpolate flow rates and heads between the times that are specified by the user. For example, if the user has specified a flow rate of 1 at time 0 and a flow rate of 2 at time 1, the FHB package will calculate a flow rate of 1.1 at time 0.1.
Head is the specified head in a cell.
The specified head for a time step will be the interpolated value of specified head at the end of the time step.
