The filter can be applied to multiple hosts or just a single host at one time. This is only dependent on how many rows/hosts you have selected in the grid when you apply the filter. Select all the rows in a grid, and then you can apply the same filter to all rows with one click.
The filter for a given host/row or multiple hosts/rows is saved in the grid and would be saved to a .bps file if you saved the grid to .bps file. However, if you want to apply a filter from one .bps file or grid to another grid or .bps file, you would have to manually copy/paste the filter text.
-Doug