Getting the right entries into XAML files and ⎕USING is something of a black art (not helped by Syncfusion's "casual" attitude to documentation).

Apply a recipe to this...

Namespace and Assembly are linked in the XAML definition

Assembly and .dll are linked by ⎕USING

Hence..

 xaml,←⊂'    xmlns:sftools="clr-namespace:Syncfusion.Windows.Tools.Controls;assembly=SyncFusion.RichTextBoxAdv.WPF"'

'Syncfusion.RichTextBoxAdv.Wpf,Syncfusion/4.5/Syncfusion.RichTextBoxAdv.WPF.dll'

So here's a table of namespace, assembly and .dll filename information...

Control
Namespace
Assembly
.dll
AutoCompleteSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
BarCodeSyncfusion.UI.Xaml.Controls.Barcode SyncFusion.SfBarcode.WPFSyncFusion.SfBarcode.WPF.dll
BulletGraphSyncfusion.UI.Xaml.BulletGraph SyncFusion.SfBulletGraph.WPFSyncFusion.SfBulletGraph.WPF.dll
BusyIndicatorSyncfusion.Windows.Tools.Controls SyncFusion.Shared.WPFSyncFusion.Shared.WPF.dll
ButtonAdvSyncfusion.Windows.Tools.Controls SyncFusion.Shared.WPFSyncFusion.Shared.WPF.dll
CalculatorSyncfusion.Windows.Controls.Input Syncfusion.SfInput.WPFSyncfusion.SfInput.WPF.dll
CalendarEditSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
CardViewSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
CarouselSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
CheckListBoxSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
ChromelessWindowSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
ClockSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
ColorPickerSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
ColumnChartSyncfusion.Windows.Chart Syncfusion.Chart.WPFSyncfusion.Chart.WPF.dll
ComboBoxAdvSyncfusion.Windows.Tools.Controls Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
DigitalGaugeSyncfusion.Windows.Gauge Syncfusion.Gauge.WPFSyncfusion.Gauge.WPF.dll
DocumentContainerSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
DoubleTextBoxSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
DropDownButtonAdvSyncfusion.Windows.Tools.Controls Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
FontListBoxSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
FontListComboBoxSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
GallerySyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
GroupBarSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
HierarchyNavigatorSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
IntegerTextBoxSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
LineChartSyncfusion.Windows.Chart Syncfusion.Chart.WPFSyncfusion.Chart.WPF.dll
MagnifierSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
MaskedTextBoxSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
MenuAdvSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
PDFViewerSyncfusion.Windows.PDFViewer Syncfusion.PdfViewer.WPFSyncfusion.PDFViewer.WPF.dll
PercentTextBoxSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
PieChartSyncfusion.Windows.Chart Syncfusion.Chart.WPFSyncfusion.Chart.WPF.dll
RadialMenuSyncfusion.Windows.Controls.Navigation Syncfusion.SfRadialMenu.WPFSyncfusion.SfRadialMenu.WPF.dll
RadialSliderSyncfusion.Windows.Controls.Navigation Syncfusion.SfRadialMenu.WPFSyncfusion.SfRadialMenu.WPF.dll
RangeSliderControlSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
RichTextBoxAdvSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WpfSyncfusion.Tools.Wpf.dll
RollingGaugeSyncfusion.Windows.Gauge Syncfusion.Gauge.WpfSyncfusion.Gauge.Wpf.dll
SfRatingSyncfusion.Windows.Controls.Input Syncfusion.SfInput.WpfSyncfusion.SfInput.Wpf.dll
SparklineSyncfusion.Windows.Chart Syncfusion.Chart.WpfSyncfusion.Chart.Wpf.dll
SplitButtonAdvSyncfusion.Windows.Tools.Controla Syncfusion.Shared.WpfSyncfusion.Shared.Wpf.dll
TaskBarSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
TileViewControlSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
TimeSpanEditSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll
ToolBarAdvSyncfusion.Windows.Tools.Controls SyncFusion.Shared.WPFSyncFusion.Shared.WPF.dll
TreeViewAdvSyncfusion.Windows.Tools.Controls Syncfusion.Tools.WPFSyncfusion.Tools.WPF.dll
UpDownSyncfusion.Windows.Shared Syncfusion.Shared.WPFSyncfusion.Shared.WPF.dll