The section treats assignment and subletting as opposite cases. Assignment - handing the whole lease over - requires the landlord's written consent, and that consent "may be unconditionally withheld without cause". The tenant's only protection is a trade: if consent is unreasonably withheld, the landlord must release the tenant from the lease on thirty days' notice if the tenant asks, and the statute says that release is the tenant's sole remedy. If consent is reasonably withheld, there is no assignment and no release.
Subletting is different, and only in buildings with four or more residential units. There the tenant has a right to sublet subject to the landlord's advance written consent, and that consent "shall not be unreasonably withheld". Paragraph (b) sets out exactly what the tenant must send by certified mail, return receipt requested: the term, the subtenant's name, the subtenant's business and permanent home addresses, the reason for subletting, the tenant's own address during the sublease, the written consent of any co-tenant or guarantor, and a copy of the proposed sublease.
The clock in paragraph (c) is the part that decides most of these disputes. The landlord has ten days to ask for additional information, which may not be unduly burdensome, and thirty days from the mailing of the request - or of the additional information, whichever is later - to send notice of consent or of the reasons for refusal. Failure to send that notice is deemed consent. If consent is unreasonably withheld the tenant may sublet anyway and may recover costs and attorneys' fees on a finding that the landlord acted in bad faith. A sublet that does not comply with the section is a substantial breach of the lease, the tenant remains liable on the lease throughout, waivers are null and void, and the section does not apply to public housing or to co-op proprietary leases.