English to Italian Bus/Financial Law: Contract(s)

Of course I know what a security is but its meaning escapes me in this peculiar context:
"Subsidiary and Holding Company: mean a "subsidiary" and "holding company" as defined in section 1159 of the Companies Act 2006 and a company shall be treated, for the purposes only of the membership requirement contained in subsections 1159(1)(b) and (c) as a member of another company even if its shares in that other company are registered in the name of (a) another person (or its nominee), whether by way of security or in connection with the taking of security, or (b) its nominee."