Most manual practitioners view the iliacus muscle and the psoas major muscle as one muscle. These two muscles do share something in common and that is they both insert into the same tendon. In many books you'll see them referred to as the 'iliopsoas' and are described as hip flexors. The 'iliopsoas' or even more often the psoas is blamed for a lot of back pain problems, especially when one has a difficult time straightening out. The psoas is implicated for a lot of back pain issues. The iliacus, on the other hand, is difficult to reach as it is located on the inner aspect of the pelvic bones and is often forgotten. If not forgotten, it is assumed that it does what the psoas does. So if the psoas is treated and relieved, it is assumed that the iliacus has resolved as well.
While I was going through training, I often noticed that when one would teach palpation and treatment of the 'iliopsoas,' they would only teach palpation and treatment of the psoas major muscle. The iliacus portion was never directly palpated or treated. Everyone assumed that if you treated the psoas muscle and it felt better, then the iliacus was no longer an issue. The iliacus, however, is a separate muscle that requires evaluation and treatment separate from the psoas because it is a different muscle and does not needly respond to a treatment aimed at the psoas muscle. Let me emphasize that these are separate muscles and have different innervation.
Speaking from personal experience as someone who suffers with severe back pain for over a year due to iliacus tightness, this is an important muscle that anyone who does manual medicine should assess and treat for low back pain issues. As an Osteopathic Manipulative Medicine (OMM) provider, I routinely check for tenderness at the iliacus when low back pain is an issue. Furthermore, one should never assume that because a psoas dysfunction has been resolved, iliacus dysfunction has been resolved simultaneously. Although not easily accessible, a portion of the iliacus can be palpated by pushing the inside wall of the hip bone from the front. Assessing patients regularly who have back pain, it is rare for iliacus not to be tender and sensitive even after psoas tension has been relieved. It is time we stop viewing the iliacus as simply just being part of the psoas major muscles. Helping your patients backs could depend on it.