Oracle 11g database datafile addition to ASM without '+' sign
If you miss the '+' sign, there will be several scenarios:


db_create_file_dest is set and points to ASM (i.e: +DATA):
The database will take 'DATA' as an alias and will create ASM file on the destination associated with the 'DATA' alias that will be located at the ASM +DATA root directory.
db_create_file_dest is set and points to filesystem:
It should be quite obvious that you'll have a new 'DATA' file in the location pointed at the parameter.
db_create_file_dest is not set (worst scenario):
The database will default to $ORACLE_HOME/dbs and create a datafile there with the name 'DATA'.


A final piece of advice: Set the db_create_file_dest to point to the ASM DATA disk group and remove the datafile name parameter from your command:

alter tablespace CWSC add datafile size 100M AUTOEXTEND ON;

As you may know, even if you give a name to the datafile, when it's created on ASM, that name becomes an alias and the real filename is set by the ASM instance itself.

I hope this info helps you.