Matrix of R-squared values. The column headers indicate the respective endogenous variables that is projected on a combination of exogenous variables. Example: If we have 4 regressors x1, x2, x3, x4, then the fist column of the returned matrix has 7 rows including the R-squared values of the following regressions:
x1 ~ x2 + x3 + x4
x1 ~ x3 + x4
x1 ~ x2 + x4
x1 ~ x2 + x3
x1 ~ x4
x1 ~ x3
x1 ~ x2
The second column corresponds to the regressions:
x2 ~ x1 + x3 + x4
x2 ~ x3 + x4
x2 ~ x1 + x4
x2 ~ x1 + x3
x2 ~ x4
x2 ~ x3
x2 ~ x1
and so on.