"Her clothes are unique pieces."
Translation:As roupas dela são peças únicas.
"Peças" are the parts of something that can be completed, "pedaços" are parts of something you cut, break or divide.
Are there particular places where we should use "seu" or "..dela", or are they completely interchangeable.
I feel like I should know this, but what is wrong with 'dela roupas são peças unicas'? Does dela be plural?
"Dela" is "de+ela", so your sentence means "From/of her clothes are unique pieces". Not very meaningfull. You could, however, use "seu/sua" in a similar sense like in "As suas roupas são peças únicas", but unfortunately without context, it's imposible to know if that means "his", "her", or most probably "your".
"Dela" is used both for singular and plural OBJECT, as in "roupaS dela" - here it is plural.
So annoying to be marked wrong for putting "pedacos" when Tinycards gives this as the translation for pieces. A peca is given as "the part"