The names of the OpenUI5 control libraries and the related subpackages are mapped to XML namespaces.
One of the required namespaces can be defined as the default namespace (xmlns="..."). The control tags for this namespace do not need a prefix.
The View tag is required and in the example below, the sap.ui.core.mvc namespace is defined with alias mvc. Technically, you can define any alias for namespaces. However, the convention is to use the last part of the full package name.
A control can be located in a subpackage of a control library, for example sap.ui.layout.form.Form is located in the sap.ui.layout library, but the full package name is sap.ui.layout.form. You have to specify this subpackage as a separate XML namespace, even if sap.ui.layout is already defined as namespace.
<mvc:View
xmlns:mvc="sap.ui.core.mvc"
xmlns:layout="sap.ui.layout"
xmlns:form="sap.ui.layout.form"
xmlns="sap.m">
</mvc:View>