What is the meaning of the phase state label (cr,l) appearing in ATcT thermochemical data tables?
The nearest I came to an explanation is an article (Ref. 1) by authors of the database explaining that, in the case of $\ce{H2O (cr,l)}$, the notation (cr,l) refers to the condensed state. Also, a link from the above data page explains that "cr=crystal" and "l=liquid" (thanks MaxW for hinting at the solution). 

A further update: it finally dawned on me that the website sometimes provides enthalpy data at two different temperatures ($\pu{0 K}$ and $\pu{298.15 K}$) in one line, so evidently the lower T refers to the crystalline state, the higher to the liquid state. But it remains somewhat confusing notation. In the case of an aqueous solution of $\ce{(NH4)NO3}$, it would seem to make sense to use the notation "(aq)" for the liquid state. Evidently "(l)" is synonymous with "(aq)" in the case of solutes in aqueous solution, and "(cr)" refers to a frozen dilute aqueous solution at $\ce{0 K}$. But this remains confusing, since in this table, the last entry describes the following reaction: $$\ce{(NH4)NO3 (cr,l) → [NH4]+ (aq) + [NO3]- (aq) }$$ Here a distinction is made between the hydrated (or aqueous) ions and the species $\ce{(NH4)NO3 (cr,l)}$, whatever it may be. It seems unlikely that $\ce{(NH4)NO3}$ can exist as a molten salt. It seems (cr,l) is used to signify a condensed phase of the pure substance, whichever makes sense under the given conditions. In the above reaction this is presumably the crystalline solid, and the reaction represents solubilization.

References


Ruscic et al., J. Phys. Chem. A, Vol. 108, No. 45, 2004. See the caption to Table 1.