What's the difference between Show and Hide for rows/columns (apart from the obvious)? The answer is how the module behaves when new items are subsequently added. So, if I have a module with 10 line items, and only want to see 5 by default then I could...
select the ones I don't want to see and select 'hide'....the module is now behaving as 'show everything except for those specifically hidden'
select the ones I do want to see and select 'show'....the module is now behaving as 'only show the items specifically selected to show'
Now I add a new item into the module. It will appear in the grid if I'd previously hidden other items, it wont appear in the grid if I'd previously shown some items. Getting it right at the outset means no more having to 'show all' and then respecify your hides/shows, and no more having to republish modules to a dashboard. Acknowledgments to Martin Overton for making me think about some behaviour which I'd seen many times before but never thought about what was actually going on.
Another tip with the "Show" feature is that you can easily re-order headers as you wish. It works with line items as well as with lists. Very useful when you don't want to modify the ordering in a list of modify the order of your line items in a module.
For instance you have 4 line items : LI1, LI2, LI3, LI4 but you want to display only LI4 and LI1, in this order.
So you just multi-select your line items LI4 and then LI1 (using the CTRL key) in the order you want them to appear in the grid, and then right-click>Show. The grid will show LI4 first and LI1 next.
It can be done in a module (then you can save a subsidiary view) or directly in a dashboard.
Works also like a charm with list items.