The same subject may be made definite through a name, relative construction, or demonstrative. Each provides a different route by which the hearer identifies the referent, and the selection may be rhetorically significant.
MethodExampleTypical function
Proper name (العلمية)زَيْدٌ عَالِمٌIdentify by name
Relative (الموصولية)الَّذِي نَجَحَ عَالِمٌIdentify through a clause
Demonstrative (الإشارة)هٰذَا عَالِمٌSingle out an identifiable referent
This is a basic comparison, not a rule that each form has only one meaning. A demonstrative can also praise, belittle, or express amazement, depending on the circumstances and supporting cues.