FH Alzheimer disease dementia custom

How to understand “FH Alzheimer disease dementia custom” in UKB?

https://app.genebass.org/gene/undefined/phenotype/categorical-FH_Alzheimer_disease_dementia_custom-both_sexes–custom

Analysis

N cases 53734
N controls 327258
N cases males 27779
N cases females 36279
N both sexes 65784
Heritability 0.014836675232158755

Thanks.

Shicheng