What is the position of the variables in the equation obtained from the rstool

It is part of my programme:

rstool(X12,CCU,'purequadratic')

stats = regstats(CCU,X12,'purequadratic')

stats.beta

From function stats.beta I've got 5 coefficients:

224,063250000000

-0,375900000000010

-81,9059250000000

0,0131565000000002

8,80680000000000

I don't know what position have each of them in the equation.

What is the model of equation used automatically in MATLAB?

Is it z=ax^2+bx+cy^2+dy+exy+f?

If yes why I've got only 5 insted of 6 coefficients?

I would like to ask for your help, thank you so much!

Best regards,

Mateusz