The boundaries of Kingdom Protista are not well-defined primarily because:
explicitly mentions that the boundaries of Kingdom Protista are not well-defined. This ambiguity arises because all single-celled eukaryotes are placed here, leading to overlapping characteristics. What one biologist categorizes as a simple 'photosynthetic protistan' (like a diatom or euglena), another biologist might classify as a single-celled 'plant' due to the presence of chloroplasts and photosynthetic capabilities.
Ref: NCERT Class 11 Biology > Chapter 2: Biological Classification > 2.2 Kingdom Protista