Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE Electoral_Register (electoral_register_id VARCHAR, cmi_cross_ref_id VARCHAR)
TABLE: CREATE TABLE CMI_Cross_References (cmi_cross_ref_id VARCHAR, source_system_code VARCHAR)

### Question ###
What are the register ids of electoral registries that have the cross reference source system code 'Electoral' or 'Tax'?

### Accurate SQL ###

SELECT T1.electoral_register_id FROM Electoral_Register AS T1 JOIN CMI_Cross_References AS T2 ON T1.cmi_cross_ref_id = T2.cmi_cross_ref_id WHERE T2.source_system_code = 'Electoral' OR T2.source_system_code = 'Tax'