/lib/partman/automatically_partition/80custom/choices is in ubiquity 18.04.14.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | #!/bin/sh
. /lib/partman/lib/base.sh
db_metaget partman-auto/text/custom_partitioning description
printf "custom\t$RET"
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