The Feeds module to import data is not usable without Feeds Tamper. This module ensures, for example, that the data from your import file fits into a multiple field. You can then separate the terms with a comma in one column and the Feeds Tamper will then read this comma as for each new item.
There are many other applications such as date formats that you can modify during the import, converting strings to capital letters, removing spaces, replacing words, ... It was a bit of a wait for this module and at the time of this writing, one can read in the documentation that there is no UI (utility interface) and that you can only work with code. None of that. The thing works.
Let us work out an example. I have a content type 'Products' with a title field and a taxonomy field that shows whether the product can be used indoors and / or outdoors. Since we have products that can be used for both, the taxonomy field is multiple.
We create a Feed:
The csv file with the column.
Do the mapping
After installation a tab 'Tamper' is present.
Here you can choose which content you want to adjust for import. You must first configure them with the Mapping in the Feed.
To split a field with different values use explode ...
It looks like that:
After import you see the values of the multiple field.