Just to add on to Brian’s reply:
The positionality of MU occurrences is affected by its compression options, not whether it is in a group or periodic group. If the multi-valued field is using ordinary compression, the MU occurrences will maintain positionality, regardless of whether they contain null values or not. If the MU is null suppressed, then null-valued occurrences will be dropped out of the array and the remaining not-null occurrences will be moved up in the MU. If the MU is not null suppressed (ordinary compression), then the MU count will always reflect the highest index used on an add/update, even if those occurrences contain null values.
Periodic groups have no option for suppressing/dropping occurrences (except for trailing null-suppressed occurrences). If you need to remove occurrence 1, you will need to programmatically move the other occurrences down one occurrence and null out the highest occurrence.
Also note that if using a “Details A(253)” field, the redefinition cannot be larger than the base field. In addition, any current version of Adabas and Natural support alpha fields longer than 253 bytes, so you may not need to use a MU field, just a longer alpha field (up to 16381 bytes) or a dynamic alpha field (which can be variable length up to almost 2gb).