The word "chapel" is derived from the Latin cappa or cope, refering to Saint Martin's legendary robe which became an object of veneration for early pilgrims.   A capella, or chapel might be a small structure intended to house this or other relics.   The term is also applied to the small spaces clustered around the edges of a great church intended partly for private devotional purposes and (sometimes) burial.  An individual (clergy or lay) might found a chapel by donating a source of income (often rent on a piece of property) to pay chaplains at regular times: the donor would gain from prayers said during mass.   Chapels also served the stational liturgy of the clergy.  In this stational liturgy the clergy might process out of the choir to honor a particular saint in their chapel on their feast day.   Chapels might also be adopted by one of the professional groups of the city: the drapers in the axial chapel of Notre-Dame, artists in the chapel of St. Eloi or woad-producers in the chapel of St. Nicolas in the nave.  A chapel does not have to be a separate and discrete space: note the chapels of St Sebastian and Notre-Dame du Puy, located against eastern piers in the transept arms: both chapels were seats of lay confraternities.
Amiens Cathedral was originally designed for ten chapels, seven radiating chapels housing altars mostly dedicated to saints of the area together with Notre-Dame in the center, an additional chapel in the south choir aisle and two more in the transept arms dedicated to Peter and Paul, the pillars of the Church.  However, between the 1290s and c1380 ten more chapels were added flanking the nave.