The CODM Rank.Encl_... are all required if and only if the rank is enclosed.
In that case, the Encl_FiltParamWhenOpen... parameters are necessary to avoid a discontinuity in the sound/effect as the swell box approaches the fully open position, and to define the limits towards which the filter frequencies should tend. The filter attenuations at fully open are always considered to be zero, but the '... when open ...' frequencies still need to be specified so that that the filter frequencies can tend towards the specified values (projected linearly between the '... when closed ...' values) as the swell box approaches the open position.
The 'Hauptwerk's swell box model' section in the CODM user's guide (pages 16-17 in the current v4.2.1 version) covers how the filter values are calculated for intermediate positions (i.e. by linear projection) and has very broad suggestions for how to approach setting the parameters.
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