Hi David,
These are documented pretty well https://devnet.kentico.com/docs/10_0/api/html/Methods_T_CMS_DataEngine_SettingsKeyInfoProvider.htm
SettingsKeyInfoProvider.GetValue Method (SettingsKeyName)
Return Value
Type: String
Returns "" if the key does not exist
SettingsKeyInfoProvider.GetValueFromDB Method (SettingsKeyName)
Return Value
Type: String
You will have to take care the validation part for the second one.