Logical assessment formula (LAF) was proposed for evaluations without accurate ground-truth labels (AGTL). In this paper, we reveal the principles of LAF via comprehensive theoretical analyses. From the revealed principles, we summarize the practicability of LAF: 1) LAF can be reasonably applied for evaluations without AGTL on a more difficult task, just acting like usual strategies for evaluations with AGTL; 2) LAF can be applied for evaluations without AGTL from the logical perspective on an easier task, unable to be acting like usual strategies for evaluations with AGTL. Experimental results and analyses of LAF applied on tumour segmentation for breast cancer support the practicability of LAF summarized from the revealed principles.