About LEI

LEI (Legal Entity Identifier) is 20-digit alpha-numeric international identifier of legal person issued in accordance with the international standard ISO 17442.
•Any legal entity could apply for LEI.Under EU regulations, any natural person performing trade or other business is obliged to use LEI. Any unique party that is legally or financially responsible for transactions of financial instruments or which has a legal right under its jurisdiction to enter into legal relationships independently is considered to be a legal person for the purposes of using the LEI under EU regulations.
•The possibility that two different entities have identical LEI identification code or that one entity has two different LEI codes is eliminated.  
• LEI code ensures unique identification of entities worldwide, high quality of reference data and integrity of systems.
• On 09 October 2017, ESMA has published a Briefing on the number of EU regulations under which are necessary to obtain and use LEI:

Legal framework  Persons required to use LEI
EMIR Counterparties to derivates contracts as well as beneficiaries, brokers, central counterparties (CCPs) and clearing members
MiFID II / MiFIR

Investment firms that execute transactions in financial instruments

The clients on whose behalf the  investment firm executes ransactions, when the client is a legal entity

The client of the firm on whose behalf the trading venue is reporting under par. 26 (5) MiFIR, when the client is a legal entity

The person who makes the decision to acquire the financial instrument, when this person is a legal entity e.g. this includes investmest managers acting under a discretionary mandate on behalf of its underlying clients

The firm transmitting the order 

The entity submitting a transaction report (i.e. trading venue, ARM, investment firm)

the issuer of any financial instrument listed and/or traded on a trading venue

MAR The issuers of financial instruments, entities involved or reporting in suspicious transactions
CRR Credit and financial institutions
AIFMD Funds and fund managers
Solvency II Pension funds and insurance companies
CSDR Central securities depositories and its participants 
Transparency Directive The issuers of financial instruments listed on Regulated Markets
SFTR Parties involved in securities financing transactions and beneficiaries of the  rights and obligations arising from these
Regulation on the prospectus  The issuers of securities 

You can find more information here.