With the traditional "master boot record partition scheme", a hard disk can
only have four "primary partitions". However one (and only one) of them can
be an "extended partition" that can contain as many "logical partitions" as
you wish, hence no real limitation. When using the system, it makes no
difference. The order of the partitions on a SSD makes no difference too.
On a HDD, reads/writes to partitions at the end of the disk are a tiny little
bit faster (because it spins less)... but you will probably never notice the
difference.
The three partitions you created (according to the first screenshot) are
OK... but not great (more on that in the next paragraph). You can use them
in a "Something else" type of installation: right after choosing that type,
you only need to indicate that / will be mounted on /dev/sdb1 and /home will
be mounted on /dev/sdb6. To do so, there are drop-down menus. To find them
click (maybe right-click, I forgot the exact process) on each of these two
partitions. You can click their "format" check box too.
All that said, I would not put the partition for user data on that /dev/sdb.
It is too small: with 97 GiB for /home, you and the other users of the
system will soon run short of space in your "home folders". I would create a
larger (the larger the better) XFS partition on another drive. In Trisquel's
installer, you would then indicate that /home will be mounted on that new
partition. You can keep /dev/sdb6 to store data. Or you can delete it and
keep the liberated space unallocated (if, in the future, you want to install
a second GNU/Linux system, you would put its root partition there, and
indicate the same /home as Trisquel's, without formatting!).
