Quranic Terminilogies (Anzal And Tanzeel)
Both words mean more or less the same same thing as per my understanding. A verb can be used both for the beginning of an action (e.g. initiation of revelation ) and for completion of action (e.g. completion of revelation ). It is the sentence structure and context which decides the stage at which that particular verb is ought to be used. Like in اِنَّاۤ اَنۡزَلۡنٰہُ فِیۡ لَیۡلَۃِ الۡقَدۡرِ , here اَنۡزَلۡنٰہُ refers to start of revelation of Quran and in تَنَزَّلُ الۡمَلٰٓئِکَۃُ وَ الرُّوۡحُ فِیۡہَا بِاِذۡنِ رَبِّہِمۡ , here تَنَزَّلُ refers to a complete action. Irfan Shahzad Sahab ( @Irfan76 ) can clarify it further.