You can find documentation for the Reactome data model here. Sidebar on the left shows the hierarchy of Reactome classes. The number of instances of this class is shown in square brackets and is hyperlinked to a page listing all instances in this class. The main panel shows attributes of the selected class. Own attributes, i.e. the ones which are not inherited from a parent class are indicated in colour. '+' in 'Cardinality' column indicates that this is a multi-value attribute. 'Value defines instance' column indicates the attributes the values of which determine instance identity and are used to check if an identical instance has been stored in the database already. 'ALL' indicates that that all of the values of a given attribute must be identical while 'ANY' shows that identity of any single value of a given attribute is enough. Of course, if the identity is defined by multiple attributes each of them has to match.