The short answer is "whenever possible."
HEAVY.AI uses a Dictionary Coder to optimize storage and performance. When you import data with Immerse, HEAVY.AI scans text-based fields for duplicate values. If 70% of the values appear more than once, the field is defined as TEXT ENCODING DICT. A dictionary table stores the common values, while the database stores reference to the full values. This can result in huge savings in storage and processing time.