A host application is the application in which a form exists at any given time. For example, if you are using LiveCycle Forms ES to render a form in HTML format, then during the pre-rendering process the host application is LiveCycle Forms ES. Once you render a form and view it in a client application such as Acrobat, Adobe Reader, or an HTML browser, then the client application becomes the host application.
LiveCycle Designer ES includes a scripting model that provides scripting properties and methods for directly interfacing with a hosting application. For example, you can use the properties and methods in the host scripting model to provide PDF page navigation actions in Acrobat or Adobe Reader, or you can use the importData method to load data into your form.
xfa.host.property_or_method
Using the host scripting model properties and methods, you can retrieve information and execute actions that are not otherwise accessible through calculations and scripts. For example, you can retrieve the name of the host application (such as Acrobat), or advance the current page on an interactive form. The following table lists the properties and methods that are available for the host scripting model.
See also 

About working with a host application