In SAPUI5, a Custom Control is a UI element that is created by extending an existing control or creating a new control from scratch. Custom controls allow developers to create reusable UI elements that encapsulate complex logic and behavior.

To create a custom control in SAPUI5, follow these steps:

1. Create a new control by extending an existing control or creating a new control from scratch. To extend an existing control, use the "extend" method of the control class. For example, to create a custom button control, extend the sap.m.Button class as follows:

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sap.ui.define([
    "sap/m/Button"
], function(Button) {
    "use strict";
    return Button.extend("my.namespace.CustomButton", {
        metadata: {
            properties: {
                text: { type: "string", defaultValue: "Custom Button" }
            },
            events: {
                press: {}
            }
        },
        init: function() {
            // Initialize control properties
        },
        renderer: function(oRm, oControl) {
            // Render control
        }
    });
});

1. Define the control properties and events in the "metadata" section of the control. Properties are used to set the state of the control, while events are used to handle user interactions.

2. Implement the "init" method to initialize the control properties.

3. Implement the "renderer" method to render the control. The renderer method is responsible for creating the HTML representation of the control.

4. Use the new control in your application by instantiating it and adding it to the UI tree. For example:

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var oCustomButton = new my.namespace.CustomButton({
    text: "Click me"
});
oCustomButton.attachPress(function() {
    // Handle button press event
});
this.getView().byId("customButtonContainer").addContent(oCustomButton);

In this example, we create an instance of the custom button control and attach a press event handler to it. We then add the control to a container element in the UI.

By following these steps, you can create custom controls in SAPUI5 that encapsulate complex logic and behavior and can be easily reused across your application.