The attributes are used in the Product List Toolbar on the frontend.
_current_grid_direction holds data as to whether the product list is currently sorted in Ascending or Descending direction.
_current_limit holds data as to how many items show on the current page, aka, per page limit of products to show.
You can see these items in use in the class
_current_grid_order is used in
Mage_Catalog_Block_Product_List_Toolbar::getCurrentOrder(). That function is called in the following functions in the same class:
_current_grid_direction is used in
Mage_Catalog_Block_Product_List_Toolbar::getCurrentOrder(). That function is called in in the following functions in the same class:
That function is also called in the following files:
If you start stepping through the code from toolbar.phtml you should be able to see it in action.