Applying for extend visitor visa, two length options confusing me...???

[realmj]

Hello ,
I found there are "less than 6 month" and "more than 6 month" options when doing online extend visitor visa.
Can anybody explain what 's the difference between them?

As everybody say the 6 months is the max. length per entry/extending term, can we apply for more than 6 months here ? if we choose more than 6 month, would be more possibility to be rejected as the potential possibility of permanent stay?

thanks!
regards