The global attributeitemtypespecifies the URL of the vocabulary that will be used to define itemprop's (item properties) in the data structure. itemscope is used to set the scope of where in the data structure the vocabulary set by itemtype will be active.
Google and other major search engines support the schema.org vocabulary for structured data. This vocabulary defines a standard set of type names and property names. For example, MusicEventindicates a concert performance, with startDate and location properties specifying the concert's key details. In this case, MusicEventwould be the URL used by itemtype, with startDate and location as itemprop's which MusicEvent defines.
The itemtype attribute must have a value that is an unordered set of unique tokens which are case-sensitive, each is a valid and absolute URL, and all defined to use the same vocabulary. The attribute's value must have at least one token.
The item types must all be types defined in applicable specifications (such as schema.org), and must all be defined to use the same vocabulary.
The itemtype attribute can only be specified on elements which have an itemscope attribute specified.
The itemid attribute can only be specified on elements which have both an itemscope attribute and an itemtype attribute specified. They must only be specified on elements with an itemscope attribute, whose itemtype attribute specifies a vocabulary not supporting global identifiers for items, as defined by that vocabulary's specification.
The exact meaning of a global identifier is determined by the vocabulary's specification. It is left to such specifications to define whether multiple items of the same global identifier (whether on the same page or different pages) are allowed to exist, and what processing rules for that vocabulary are, with respect to handling the case of multiple items with the same ID.