Rodrigo, 
Since v15.3 we have both Display-String and Description-String:
 
- Display-string has a maximum length of 20 characters and does nos accept spaces.  
- Description-string has a maximum length of 255 characters and accepts spaces.
Show port  info will display the display-string.
Port      Flags               Link      ELSM Link Num Num  Num   Jumbo QOS     Load
                              State     /OAM UPS  STP VLAN Proto Size  profile Master
=====================================================================================
PortDisp> Em------e--fMB----- active    - / -  5    0    2   0   9216  none    
=====================================================================================
Show port  info detail will display both the display-string and the description-string
Port:    1(
PortDisplayString😞
    Description String: "
This Is The Port Description String"
    Virtual-router:    VR-Default
    Type:        UTP 
    Random Early drop:    Unsupported
    Admin state:    Enabled with  auto-speed sensing  auto-duplex
    Link State:    Active, 100Mbps, full-duplex
    Link Ups:       5        Last: Thu Nov 06 17:17:53 2014
    Link Downs:     4        Last: Thu Nov 06 17:17:47 2014
Whatever you write in the MIB variable ifAlias (1.3.6.1.2.1.31.1.1.1.18) will show both in Description String and in Display String (truncated to 20 chars).
If you only set Display-String, it will appear in ifAlias. But if you set Description-String, it will replace the Display-String value in ifAlias. If you then change Display-String, ifAlias won't change, unless you unconfigure Display-String...
Confusing, I know...
I've been playing with NetSight MIB Tool and EXOS v15.6.1.4