OK, thanks for the screenshots!
What you are willling to achieve requires a pretty-consuming expression.
Here is my suggestion:
IFS(
IN(LEFT([Model Number],4), Data Catagories[Model [String]],
LOOKUP(LEFT([Model Number],4), "Data Catagories", "Model [String]", "Category"),
IN(LEFT([Model Number],3), Data Catagories[Model [String]],
LOOKUP(LEFT([Model Number],3), "Data Catagories", "Model [String]", "Category"),
IN(LEFT([Model Number],2), Data Catagories[Model [String]],
LOOKUP(LEFT([Model Number],2), "Data Catagories", "Model [String]", "Category")
)
or:
IFS(
IN(LEFT([Model Number],4), Data Catagories[Model [String]],
LOOKUP(LEFT([Model Number],4), "Data Catagories", "Model [String]", "Category"),
IN(LEFT([Model Number],3), Data Catagories[Model [String]],
LOOKUP(LEFT([Model Number],3), "Data Catagories", "Model [String]", "Category"),
TRUE,
LOOKUP(LEFT([Model Number],2), "Data Catagories", "Model [String]", "Category")
)
For reference:
IFS() - AppSheet Help