我在用stata里用xtgls面板数据回归时，这些命令选项不大懂什么意思，
请高手指点：

Fit panel-data model with heteroskedasticity across panels
        . xtgls invest market stock, panels(hetero)
    Correlation and heteroskedasticity across panels
        . xtgls invest market stock, panels(correlated)
    Heteroskedasticity across panels and autocorrelation within panels
        . xtgls invest market stock, panels(hetero) corr(ar1)

请问各个命令Fit panel-data model with heteroskedasticity across panels
和Correlation and heteroskedasticity across panels
和Heteroskedasticity across panels and autocorrelation within panels
各什么意思呢，不大懂。