Ring & Pinion gears are commonly known as Crown Wheel & Pinion gears outside the U.S.
Part numbers provide more information than you might think at first glance. Typically the Differential associated with the gears is listed first (D44), then the ratio(513), followed by a qualifier (T) representing a Thick gear and completed with the designation (NG) for Nitro-Gear. D44-513T-NG = Dana 44, 5.13 Ratio, Thick, Nitro Gear & Axle
Some common indicators to look for:
R - Denotes a Reverse Rotation gear, commonly found in front axle applications. Reverse gears have ring gear teeth that curve to the left and a pinion that intersects the ring gear above the axle centerline.
T - Denotes a Thick* gear, used to circumvent the purchase of a new carrier. Some OEM Manufacturers designed a 'carrier break' into their product lines. These carriers breaks are often referred to as a numerical carrier series (e.g. 3 series or 4 series depending on ratio range). In the case of the Dana 44 the break occurs between gear ratios 3.73 & 3.91. (3.73 and numerically lower gears/ 3.91 and numerically higher gears). In this example if you own the 3.73 & down carrier, you would purchase the 5.13 Thick gear. The thick gear compensates for the difference in deck height saving you from needing to buy both a regular 5.13 ring & pinion and a 3.91 & up carrier.
*Some thin gears also exist to achieve a similar result.