This system can be observed when studying the utterances of the learner who attempts to produce meaning in their L2 speech; it is not seen when that same learner performs form-focused tasks, such as oral drills in a classroom.
Interlanguage can be variable across different contexts; for example, it may be more accurate, complex and fluent in one domain than in another.
It can "fossilize", or cease developing, in any of its developmental stages.
The interlanguage rules are claimed to be shaped by several factors, including L1-transfer, previous learning strategies, strategies of L2 acquisition (i.e., simplification), L2 communication strategies (i.e., circumlocution), and overgeneralization of L2 language patterns.
On the other hand, those who approach it from a sociolinguistic or psycholinguistic orientation view variability as an inherent feature of the learner's interlanguage.