I believe you're looking for the word "use".
I know that seems overly simple, but I've been in construction for almost 40 years and the designation of a buildings purpose, residential, commercial, retail, office etc. is called its "use"
I assume you are referring to structures that are left over and serve no apparent purpose. These are called vestigial structures.
They are purposeless structures
These structures are called vestigial structures. They are remnants of ancestral features that no longer serve a clear purpose in the organism's current form. Examples include the human appendix and whale leg bones.
Structures that are similar are called analogous structures. These structures may serve similar functions but have evolved independently in different species.
Homologous structures.
chromosomes
homologus structures
It is either homologous structures or homozygous structures. Embryological structures are when different species of animals look similar in the earliest stage of development and Analogous structures are when animals look different but their function is basically the same. So just look up homologous structures and homozygous structures in your Bio book!!
The colorful, leaf-like structures of flowers are called petals.
Similar structures that evolved independently are called analogous structures, or analogies.
The threadlike structures are called rhizoids.
NUCLEUS