Custodians are the institutions which hold your securities for you and provide some related services.
Some will have various arrangements and relationships with exchanges and broker-dealers, and some may do everything in-house; such things have bearing on what your investment options are, how much equity you must have for margin, what kind of fees you pay for various services, and so on.
Different custodians tend to structure their fees and services to a particular type of clientele or a particular account size. You may outgrow the custodian you have, or you may discover that there is a better, more affordable option for an account like yours.
If you trade often, it might be a good idea to establish your account with a discount brokerage (to reduce commissions), and if you have a portfolio of very significant size which requires a full spectrum of asset classes, it might be a good idea to have them at a large, full-service investment house.
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