Very, very, important function.
By far the most difficult and delicate procedure within this kind of software.

Here flexibility is an asset, if you are not able to shape the shipped filters you must completely rely on them.

First of all flexibility means: "IF ... THEN", i.e. to be able to set conditions. Conditions can be set also when "IF ... THEN" is not explicitly stated, because selection of  predesigned subordinate options often implies conditional commands. Predesigned subordinate options are almost always present in BMS, something like: "if there are more than 3 authors, display only the first and add [et al.]", is more than one simple "IF ... THEN", and it has already been handled by programmers, the code is hidden, you only choose one or more options. Parsing is another example of imposing conditions. Parsing means fragmenting one field to send its chopped contents to different fields. Parsing is important, often used with the 'source' field for Journal title, volume, issue, date, pages.

It is crucial to be able to handle several formats (tabbed, delimited, tagged in various ways).

Relevant issues: varying structure and position of field tags, occurrences separator for multi-value fields, wrapping lines.

None of the reviewed packages is able to import records formatted in ISO 2709 format, including MARC bibliographic records, but nowadays MARC records are most often displayed, captured and converted in the tagged (labelled) format. One way to get them is via the Z39.50 search and retrieve protocol.  The latest trend consists in letting the package catch bibliographic data displayed on the web and swallow it directly within its own database,  by direct "grasping", "copy to", drag-and-drop, without having to save or to export a file and then import it in separate steps.

The general tendency is to offer the user hundreds of ready-made filters. I would never trust any of them without double-checking what they do with the data I am interested in, which often implies working with very few records printed out on paper, a pencil and a certain amount of attention: genius is not required. At this moment it is essential that import filters are customizable.

See also Testing.