问在此select语句中，N和E表示什么

EN

foreach ($computer in $computername) {

      $context = New-Object -TypeName System.DirectoryServices.AccountManagement.PrincipalContext -ArgumentList $ctype, $computer

      $idtype = [System.DirectoryServices.AccountManagement.IdentityType]::SamAccountName

      $group = [System.DirectoryServices.AccountManagement.GroupPrincipal]::FindByIdentity($context, $idtype, 'Administrators')

      $group.Members | select @{N='Server'; E={$computer}}, @{N='Domain'; E={$_.Context.Name}}, samaccountName
}