blocking g/l accounts

Hi, Experts

can u explain the variations of these blocking levels.

when we use FS00 and click on lock you have two tabs there

i. Block in chart of account

(a) Blocked for creation :

(b) Blocked for posting: if i click this check box it will block for all company codes.

(c) Blocked for planning

ii. Block in company code.

(a) Blocked for posting: if i click this check box it will block for one company code.

I want to know what is blocked for creation & blocked for planning, when we have to click this check box..