Rounded Corner TextBox in WPF

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C#

WPF

hi every body
i search the web for TextBox with rounded corners and find a xaml code like below:

 <Style TargetType="{x:Type TextBox}">
            <Setter Property="HorizontalContentAlignment" Value="Center"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate >
                        <Border removed="{TemplateBinding Background}" x:Name="Bd" 
BorderThickness="2" CornerRadius="5" BorderBrush="#FFF9EAB6">
                           

 <ScrollViewer x:Name="PART_ContentHost" />


                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsMouseOver" Value="True">
                                <Setter TargetName="Bd" Property="BorderBrush" Value="#FFC7B0B0"/>
                            </Trigger>
                            <Trigger Property="IsKeyboardFocused" Value="True">
                                <Setter TargetName="Bd" Property="BorderBrush" Value="#FFC7B0B0"/>
                                <Setter Property="Foreground" Value="Black"/>
                            </Trigger>
                            <Trigger Property="IsKeyboardFocused" Value="False">
                                <Setter Property="Foreground" Value="#FFC7B0B0"/>
                            </Trigger>
                            <Trigger Property="Width" Value="Auto">
                                <Setter Property="MinWidth" Value="120"/>
                            </Trigger>
                            <Trigger Property="Height" Value="Auto">
                                <Setter Property="MinHeight" Value="27"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>

                </Setter.Value>
            </Setter>
        </Style>

i want to find out what is

<ScrollViewer x:Name="PART_ContentHost" />

in detail and why not properly work my template if delete this line from it,
please tell me completely in detail.
thanks alot.