While working on Segments, even if you have enabled a custom field, it will not be shown as an option in the Segment(beta).
e.g:

The segmentation can only happen on profile fields with known answers like a dropdown value or a number range value. If a profile field is just free text or some other type of text input it can’t be used for segmentation since there are no known values to a segment on.
e.g:

The segmentation can only happen on profile fields with known answers like a dropdown value or a number range value. If a profile field is just free text or some other type of text input it can’t be used for segmentation since there are no known values to a segment on.
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