LTE-BL/CE (LTE-M1) - CE Level
CE Operation is categorized into four difference levels. These for groups are devided into two groups and each of the group belong to two different Operation Mode as below.
Each of the level differs mostly in RACH and Paging process. Some of the major differences with different CE levels are :
How UE can figure out which Mode and which Level it has to apply ?
Basically Operation mode is determined by eNB and the Level within each Mode is determined by UE. The operation mode is determed by eNB and informed to UE via RRC message and the level is determined by UE. Then you may ask how UE determine the level and inform it to eNB ? UE determined the level based on the reference signal power (RSRP) it measure and inform it to eNB by PRACH resource (frequency, time, preamble) it uses.
 4G, LTE-Advanced Pro and The Road to 5G by Erik Dahlman, Stefan Parkvall, Johan Skold
 3GPP TSG RAN WG1 Meeting #83 R1-156698 : MPDCCH for Paging at multiple CE Levels
 3GPP TS 36.213 V13.2.0 (2016-06) : Physical layer procedures (Release 13)