What determines whether an Oracle object (table, view, etc.) is required to be qualified with a schema name (for example, schema.table\_name, or schema.view\_name, etc.)? At times I am able to access a remote objects (via a DB link) without having to qualify the schema, but other times, I receive an error stating that the "table or view doesn't exist", and to correct this, I must qualify the schema name. 

I am aware that it is a best practice to always qualify a schema name, but I am just curious why I am able to access certain remote objects without a qualified schema, and others only with a qualified schema.