Self aligning roller bearing 21318-E-SKF

What is a self aligning bearing?
 A spherical bearing, or swinging bearing is a bearing that is able to accept that the spindle is slightly out of alignment.   These bearings are regularly located in grinders or agricultural machines, regularly placed in "cast iron SN" bearings, with a shim washer and seals specific to these bearings.   Attention, there are 2 important variants:   • The cylindrical bore bearing, which will be mounted directly on the shaft, in force 
• The tapered bore bearing, mounted with a sleeve, on an axis smaller than the bore of basis of turnover.   The tapered bore is indicated by the letter "K" in the reference. 
What does the letter "K" mean in the reference?
 The letter K means that the bearing has a tapered bore. It should therefore almost always be mounted with a conical sleeve. Attention, this bearing is generally mounted on an axis of 5mm less than the dimension indicated for the bearing.