What is the difference between the following function?

Hi all,
I use the two following function to simulation C Varactor using ADS.
1.C_Varactor=-1/(2*pi*freq[0,0]*imag(Z11[0]))
2.C_Varactor=-1/(2*pi*freq*imag(Z(1,1)))
Get two different data display as attached display.And what is the [0.0] of freq[0,0] and what is the difference between freq[0,0] and freq in Equation?.and what is the difference between Z11[0] and Z(1,1)?
Thank you,

Z[0] means 0th index.So, there is a vector definition for that.

Thank BigBoss for your reply.And What about the vector definition of freq[0,0].I can not find in help document.Anybody can help me?