Check No Match FALSE on Mvl when auto primary is default

 Hi,

As per oracle documentation, if the check no match property is set to FALSE then 

1. If there is a NULL or invalid value in the foreign key field siebel performs a secondary query and if it founds no matching records set the foreign key as 'nomatchrowid'

Now i have following questions.

1.What does an incalid value inforeign key column mean? How does siebel know in advance if value is invalid?

2. What does 'matching record' mean here

3. Finally what if the auto primary is set to default and siebel performs a secondary query, in that case will it update the foreign key as the default record if the alredy key value is NULL or INVALUD