What is the difference between ja_JP.UTF-8 and ja_JP.utf8?

Issue

• There is a difference in the locale set to the locale definition name and the environments. What is the difference between ja_JP.UTF-8 and ja_JP.utf8?

# locale  
LANG=ja_JP.UTF-8  
LC_CTYPE="ja_JP.UTF-8"  
LC_NUMERIC="ja_JP.UTF-8"  
LC_TIME="ja_JP.UTF-8"  
LC_COLLATE="ja_JP.UTF-8"  
LC_MONETARY="ja_JP.UTF-8"  
LC_MESSAGES="ja_JP.UTF-8"  
LC_PAPER="ja_JP.UTF-8"  
LC_NAME="ja_JP.UTF-8"  
LC_ADDRESS="ja_JP.UTF-8"  
LC_TELEPHONE="ja_JP.UTF-8"  
LC_MEASUREMENT="ja_JP.UTF-8"  
LC_IDENTIFICATION="ja_JP.UTF-8"  
LC_ALL=

# locale -a | grep ja_JP  
ja_JP  
ja_JP.eucjp  
ja_JP.ujis  
ja_JP.utf8  
japanese  
japanese.euc