Intihāl chiefly concerns claiming another poet's verse as one's own; verbal alteration concerns modifying the inherited expression. One person may reproduce an entire composition and claim authorship; another may preserve an idea but change the language.
These are not evaluated by one test. The first centers on text and attribution; the second on the specificity of the thought and the extent of transformation.
The distinction keeps copying, authorship claims and semantic resemblance analytically separate.