The mapping table is never blank when it opens. Three things have already run, and what you're looking at is a proposal to check rather than a form to fill in.
Where this is in the app
Dashboard → Import assets — or, from a return, Assets tab → Add assets → Bulk import
The mapping table is the Map Columns step of the import: step 2 with your own file, step 3.5 on the vendor route. See Mapping columns, and what the step does.
1. Your file's vendor was recognized, where it could be
A file that fingerprints as a known vendor export arrives with its mapping already applied. That vendor's columns are a known quantity, so there's usually nothing to do.
2. Column names were matched against a synonym table
For a file with no vendor fingerprint, header names are matched against the names we know. "Book Life", "Useful Life (Months)" and "GAAP Life" all land on the same field.
This pass is deterministic. No AI, no cost, same answer every time.
3. The AI looked at what was left
Anything the first two passes didn't settle gets an AI suggestion, each carrying a confidence level and a reason you can read.
The AI suggests, it doesn't decide
A suggestion is shown with its confidence and its reasoning, and you accept or override it.
The point of showing the reasoning is that you can disagree with it cheaply. A column named "Cost" in a file that also has "Net Cost" is exactly the kind of call worth a second's attention, and exactly the kind a silent auto-map would hide from you.
A recognized vendor file sometimes goes straight through
On the vendor route the mapping step is skipped only when two things are both true: every column resolved to a field, and our AI cross-check agrees with the vendor's own mapping. Then the file goes straight to preview, and you can still return with Back to mapping.
If either is untrue you get the step. That includes a complete vendor mapping the AI reads differently, which is common enough that a clean export still stops here.
That disagreement is the point rather than a nuisance. It appears on the column as, for example, "Fixed Assets CS (Thomson Reuters) adapter mapped this column to OriginalMethodCode; AI suggests Method", with a Use AI mapping button. Keeping the vendor's answer is a legitimate choice, and it is what happens if you do nothing.