SG-r01 Ame-no-Habakiri (Adaptor: Tsubasa)
SG-r02 Ichaival (Adaptor: Chris)
SG-r03' Gungnir (Adaptor: Hibiki)
SG-x00 Airget-lahm (Adaptor: Maria)
SG-i01 Shul Shagana (Adaptor: Shirabe)
SG-i02 Igalima (Adaptor: Kirika)
"SG" stands for "Symphogear." The "r" and "i" stand for "regular" and "irregular," respectively. This is based upon various facts and circumstances surrounding their development.
The "x," which is unique to Airget-lahm, indicates it was created from an "unidentified relic," as it was unable to be classified after analysis.
Furthermore, the extra apostrophe after Gungnir’s ID number is to separate it from another unit developed from a fragment of the same relic. It also signifies that this unit was developed after the original "03." On the whole, whenever the Symphogears are discussed, their model number is usually abbreviated or omitted entirely in favor of simply referring to them by the base relic.
As an addendum, the below list includes the model numbers for the Symphogear units that have been lost:
SG-r03 Gungnir (Adaptor: Kanade -> Hibiki)
SG-i03 Shenshoujing (Adaptor: Miku)