Always

1. Go to the plan details page, Mappings tab, on a existing plan
2. Select 'standard' on the target storage mapping drop down list, on edit mode 
3. Update mappings button will remain disabled

If before selecting "standard", user selects another target, then select "stardard" button will be enabled but previous target will be saved 

"standard" appears as a target storage options but it can't be saved, no error message is displayed

If before selecting "standard", user selects another target, then select "stardard" button will be enabled but previous target will be saved 

"standard" should not appear as an option if it's not allowed to be saves, alternatively an error message should appear when it's selected

On that case that "standard" selected afterwards, the selected target should be saved, not the previous one.

"standard" storage option does not appear as an option on plan creation wizard.