Cross-project lookups are a way to access the data sets of a project from within a different project on the same instance. The online help for Apptio Studio contains detailed information for common use cases.
First, navigate to Studio Guide > Formulas and Functions > Syntax for Formulas and Functions. There are two relevant Subtopics: "Data lookup" (for just grabbing a value from somewhere else) and "Referencing column names" (for using the value in a function). Syntax for cross-project lookups are covered in both of these topics.
External data lookup - different project
=project name!{table}:{column1}[column2 selector]
| the same data set or table | {column name} |
| a different data set or table | {data set name}:{column name} |
| a data set in a different project in the same domain | project name!{data set name}:{column name} |
| a data set in a different project in a different domain | domain name:project name!{data set name}:{column name} |