What do the 3 numbers mean in manual partitioning of HDD in Ubuntu preseed?

I see in official documentation this:

#d-i partman-auto/expert_recipe string                         \
#      boot-root ::                                            \
#              40 50 100 ext3                                  \
#                      $primary{ } $bootable{ }                \
#                      method{ format } format{ }              \
#                      use_filesystem{ } filesystem{ ext3 }    \
#                      mountpoint{ /boot }                     \
#              .                                               \

What is this 40 50 100? What do these numbers mean? What size will in /boot partition ? I have not found an explanation in the documentation? C

The three numbers are minimum size, priority, and maximum size.

If there is enough disk space to create this partition and all the partitions with a higher priority value with their maximum sizes, and still enough to create all defined lower-priority partitions with at least their minimum sizes, then you'll get the maximum size.

Basically, partman-auto starts with taking the minimum sizes of all the partitions. If the disk is not big enough to satisfy them, the partitioning fails and installation stops there.

If the disk is at least big enough to implement the minimum sizes, then any remaining disk capacity goes towards extending the highest-priority partition; once it reaches maximum size, then the next highest-priority partition gets more space; and so on.

When either all the available disk space on the target disk is used, or all the partitions have reached their maximum sizes, partman-auto has found the right partition sizes and actually does the partitioning.

Ubuntu uses the same installer technology as Debian does, so here's as official documentation as it gets: https://github.com/xobs/debian-installer/blob/master/doc/devel/partman-auto-recipe.txt