number seven
柒 is a variant form for the number seven, used in certain contexts such as legal or financial documents to prevent forgery. Its standalone meaning is simply 'seven'.
柒 is a variant form of 七 (seven), created by adding 木 (tree) and 氵 (water) to make the character more complex and harder to alter in official documents. The exact historical development of this particular variant is uncertain.
The top part looks like water 氵 over a stylized seven 七, and the bottom is a tree 木. Imagine writing the number seven on a wooden tablet with water to make it official and unchangeable.
For シツ, picture a sheet of paper where the formal number seven is written: sheet -> シツ, and the sheet prevents anyone from changing the seven.