Reply To: parameter: how to require either one or the other but not both

You can do this using parameter sets. By putting each parameter in one set and not the other, PowerShell will take care of the rest. You may want to define a default parameter set, as well:

function YourCmdletName
{
    [CmdletBinding(DefaultParameterSetName='ByUserName')]
    param (
        [Parameter(Mandatory = $true, ParameterSetName = 'ByUserName')]
        [System.String]
        $UserName,

        [Parameter(Mandatory = $true, ParameterSetName = 'ByEmployeeId')]
        [System.String]
        $EmployeeId
    )

    # In the function's code, you can either test for whether $UserName and/or $EmployeedId are null, or
    # check $PSBoundParameters.ContainsKey('UserName'), or check $PSCmdlet.ParameterSetName to see whether it
    # is set to 'ByUserName' or 'ByEmployeeId'.
}