Scatter plot of the I([CII])_i versus I([NII])_i integrated intensities. Data points are shown as open black circles. The Abel (2006) predictions for the [CII] emission contribution from H II regions, for metallicities 1.0 Z_⊙ and 0.1 Z_⊙ (Eqs. (1) and (2) in their work), and the Heiles (1994) linear correlation between I([CII]) and I([NII]) for the extended low-density warm ionized medium (ELDWIM; Eq. (12b) in their work) are shown as dashed, dotted, and dotted-dashed lines, respectively. The model predictions in this work, using different [C/N] abundance ratios normalized to the Galactic disk value in Simón-Díaz & Stasińska (2011) are displayed in colors. Metallicities 1.0 Z_⊙ and 2.0 Z_⊙ are shown asfilled and open ■ and ▴ symbols for log n(H) = 3.5 and 2.5, respectively. The symbol size is proportional to the ionization parameter log₁₀ U = −1, −2,−3, and −4.