In trademark practices of many countries, the term of “parts and fittings” is not acceptable for specifications of goods or services for trademark applications.
That is because the term is too broad or vague to determine the scope of protection of the trademark.
However, the term is acceptable in the Japanese trademark practice.
In the practice, for example, we can designate the goods “smartphones and parts and fittings thereof” in international class 9.
This “parts and fittings thereof” can cover any components of a smartphone and accessories which are used along with a smartphone.
In addition, “parts and fittings thereof” shall be fallen in the same similar group code, which is like a subclass to determine the similarity of goods and services, as “smartphones”.
Of course, I recommend you to use a concrete term “smartphone cases” if you want to protect this item, for example. But, this practice may be useful when you are not sure of concrete items for which the mark is used.