How to use CellValue() function

Hello,

I need to create a member formula to test if account value is negative. I never used CellValue() function. I'm having difficulty figuring it out.

Here is my member formula. I'm testing if AlllocatedBSPlug account is <0. If it is the case, I want to pull the value of the AlllocatedBSPlug account.

CASE
   When is ([Accounts].currentmember,[AlllocatedBSPlug]) then
     Case
      WHEN CellValue() <= 0 THEN

AlllocatedBSPlug

end
end

Any help will be greatly appreciated!