What is detected
Sheet names, header row, sample values, and likely field types (text, number, date, and so on). Excel formulas on the sheet can become formula fields instead of frozen numbers.
When to go manual
If the file has merged headers, two tables on one sheet, or columns that are really relations, choose manual mode and map each column yourself. Autodetect will not silently invent a structure you did not confirm.
Relations
A column that looks like “Client” or client_id can become a relation to another imported or existing table. Enable it on the Relations step and pick the field to match (usually name or a unique key).