The step-by-step check item type, allows you to collect your check item data according to the sequence in which it was built/created, or arranged. Take, for example, you have three check items(A, B, C), in step by step process, you can not collect data for check item C without collecting data first for check item A and B. Also, you can not collect data for check item B without collecting first for A.
See the article "How to Collect Check Items in Step by Step" for more understanding.