Org unit relation ship periods

Dear All,

Scenario:-

I have created a root or unit,under this root org unit i have created additional org units but the validity period box contains to validity periods are there.

organisational unit :

Link text string :

The above two will be displayed as Object period and relationship period on the screen.

Here my doubt is if i enter the date for organisational unit beyond the root organisational unit its allowing but in case of link text string(relationship period) its not allowing.

can any body explain the logic behind this issue.

Thanks inadvance

Regards

Ananya.K