As computing resources have become less expensive, the line between PCs and other products has blurred. Today many devices that are not PCs in the classic sense have a need to connect directly to peripherals: Printers connect directly with cameras, for example, or mobile phones may need to connect to USB headsets. These non-PCs have the computing resources to manage a USB host function, but they need to function in ways that differ from standard PC hosts. Although they will provide host capability for some devices, it's unreasonable to require them to support the full range of USB peripherals. For example, connecting a camera to a printer makes a lot of sense, but the printer manufacturers may not think it is quite as important for the printer to support a USB GPS dongle. Because this is new territory for USB, developers need a way to understand what USB functionality they need to provide and what functionality is not required. This specification defines these non-PC hosts as Targeted Hosts. A Targeted Host is a USB host that supports a specific, targeted set of peripherals. The developer of each Targeted Host product defines the set of supported peripherals on a Targeted Peripheral List (TPL). A Targeted Host needs to provide only the power, bus speeds, data flow types, etc., that the peripherals on its TPL require.
An OTG device is required to have one, and only one USB connector: a Micro-AB receptacle as defined in [Micro-USB1.01]. This receptacle is capable of accepting either a Micro-A plug or a Micro-B plug attached to any of the legal cables and adapters defined in [Micro-USB1.01]. [...] An Embedded Host is required to have one or more Standard-A receptacles as defined in [USB2.0]. Optionally an Embedded Host may also contain one or more Type-B receptacles but these must be implemented such that the user is unlikely confuse the EH with a USB hub. Since the Standard-A plug does not contain an ID pin, an EH is not able to automatically power the USB bus on plug insertion unless ADP is also supported.