Update Taxonomy type field value, the API works different on office files(.docx, .pptx etc.) and other files(.txt ,.pdf etc.)

Hi all

I know there is a way to use the SPTaxonomyField to update the Taxonomy filed value.

But for a specific tool, I use the field name to update the field value directly. The user story is as following

End-user decide to upload a file to a SharePoint library with some field values.

Now the problem is just about the taxonomy type field.

When upload a non-office file, such as a .txt file, the taxonomy field value can be updated correctly.

spitem["taxonomy field"]="term label|term guid";

But for office file, such as .docx, it won't work, and i found out it need to update a corresponding note type field of the taxonomy field. like this

spitem["taxonomy field"]="term label|term guid";
spitem["taxonomy field_0"]="term label|term guid";

Does anyone know why need to update the corresponding note type filed for office files. And is the corresponding note type field always has the field name "xxx_0"?

Thanks a lot.