The identifier is the name of an Analytica object. Each object in Analytica's global name space has a unique identifier.
The identifier is an attribute of an Analytica object. So for example, the expression:
Identifier Of Time
Identifiers must be between 1 and 20 characters in length, contain only letters A thru Z, digits 0 thru 9, and underscores, and the first character must be a letter. The capitalization of an Identifier is preserved when you enter it, so you can use "camel case" (capitalizing the first letter of each word, as in CamelCase); however, when you use an identifier in an expression, it is case insensitive. But the capitalization entered initially will be displayed whenever Analytica displays the identifier.
When you enter a title for an object, Analytica will usually automatically generate an identifier from the title, and may ask you if you want to rename it (the exact behavior can be configured on the Preference Dialog). It does this by replacing spaces and punctuation with underscores, converting Chinese character to their Pinyin romanizations, appending digits to the end to ensure uniqueness, etc. One "gotcha" to watch out for is the appending of digits when an object already exists with the same name. So, for example, if you title a variable NPV, the automatically generated Identifier is NPV1, which isn't obvious when you are viewing only the title on a diagram. In a Definition of another variable, you will need to use NPV1 in your expression. The 1 appends here because the name NPV is already used by the built-in function.
In most Analytica windows, including diagrams, tables, and the outliner, you can toggle between viewing Identifiers or Titles by pressing Ctrl+Y (on toggling Show by identifier on the Object menu). This shortcut becomes second-nature to seasoned model builders.