Parasegments differ from segments because:
Parasegments are functional genetic units defined by expression borders of pair-rule and segment polarity genes, notably engrailed stripes labeling anterior boundary of parasegment, whereas segments are morphological constrictions visible later through cuticular grooves offset posteriorly by half unit. Parasegment boundaries coincide with lineage restriction compartments maintained by lineage rather than grooves. Therefore parasegments represent genetic domains where regulatory interactions maintain identity, while segments reflect morphological boundaries. This distinction explains why wingless and engrailed interact across parasegment border but later furrow lies behind engrailed stripe.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Parasegment versus segment - genetic domains offset by lineage compartments.