Arabic is very simple language, we have names for every thing ( simple and predicted name usually ), but you can use the main form for almost all of them.
For example, هذا كِتَابٌ وهذه كُتُبٌ it is rare to hear كِتَابُ الطَّالِبِ \ كِتَابُ التَّمَارِينِ \ كِتَابُ المُعَلِّم all of them are just a book كتاب. On the same way, حَقِيبَةٌ\ حَقَائِب in stander Arabic and شنطة\ شنط in Egyptian Arabic. حقيبة ظَهر \ حقيبة تَسوّق \ حقيبة سَفَرَ also can be used but you can only say حقيبة . It is that simple.
Good luck and keep learning.