What is a non-portable IPv4 address block?

Question

For example, a Whois query for 4.4.4.0 says:

ADDRESSES WITHIN THIS BLOCK ARE NON-PORTABLE

Does the owner simply declare the block or a subnet are not up for sale? Or does non-portable have a deeper, technical meaning?

Here is the relevant section from the Whois query:

NetRange:       4.0.0.0 - 4.255.255.255
CIDR:           4.0.0.0/8
OriginAS:       
NetName:        LVLT-ORG-4-8
NetHandle:      NET-4-0-0-0-1
Parent:         
NetType:        Direct Allocation
RegDate:        1992-12-01
Updated:        2012-02-24
Ref:            http:// whois.arin.net/rest/net/NET-4-0-0-0-1

OrgName:        Level 3 Communications, Inc.
OrgId:          LVLT
Address:        1025 Eldorado Blvd.
City:           Broomfield
StateProv:      CO
PostalCode:     80021
Country:        US
RegDate:        1998-05-22
Updated:        2012-01-30
Comment:        ADDRESSES WITHIN THIS BLOCK ARE NON-PORTABLE
Ref:            http:// whois.arin.net/rest/org/LVLT

Answer 1

Does the owner simply declare the block or a subnet are not up for sale? Or does non-portable have a deeper, technical meaning?

Non-portablility isn't about sales... it's a technical term.

Address Portability

Non-portable IP addresses belong to a certain organization, and an end-user of the non-portable IP address is not permitted to discouraged from announcing those IP addresses to another organization without the owing organization SWIPing the addresses to the end-user.

The concept of portability comes from the ARIN Number Resource Policy Manual 2014.1, Section on IPv4 (emphasis mine)...

4. IPv4

 4.1. General Principles

  4.1.1. Routability

  Provider independent (*portable*) addresses issued directly from ARIN 
  or other Regional Registries are not guaranteed to be globally routable.

The reason the RIRs don't guarantee that portable addresses are globally routable is because the RIRs can't influence route filtering policies of providers.

Why some blocks are non-portable

Addresses are either allocated or assigned. If an address block is assigned to the ISP, then that ISP cannot sub-assign the addresses. Quoting ARIN Number Resource Policy Manual 2014.1, Section on 2.5

 2.5. Allocate and Assign

 A distinction is made between address allocation and address assignment, 
 i.e., ISPs are "allocated" address space as described herein, while end-
 users are "assigned" address space.

 > Allocate - To allocate means to distribute address space to IRs for 
 the purpose of subsequent distribution by them.

 > Assign - To assign means to delegate address space to an ISP or 
 end-user, for specific use within the Internet infrastructure 
 they operate. Assignments must only be made for specific purposes 
 documented by specific organizations and are not to be sub-assigned to 
 other parties.

References

Registries operate in accordance with RFC 7020, which replaced RFC 2050. Arguably, much of the structure around these restrictions comes from RFC 7020 Section 2, Goals:

2) Hierarchical Allocation: Given current routing technology, the distribution of IP addresses in a hierarchical manner increases the likelihood of continued scaling of the Internet's routing system. As such, it is currently a goal to allocate IP addresses in such a way that permits aggregation of these addresses into a minimum number of routing announcements.

End Note

You asked about "the block or a subnet"; however, ARIN deals with allocations of individual IP addresses, not subnets or blocks of addresses.

Answer 2

I am not a specialist in ARIN procedure, but here is how it works in RIPE-land:

Provider Aggregated ("PA", "non-portable") resources are blocks allocated to a Local Internet Registry (LIR) - normally the end-users ISP - parts of which the LIR assigns to its end-users. The resources thus assigned remain under the management of the LIR and stay assigned to the end-user while the assignment criteria are valid and the end-user maintains a contractual relationship with the LIR. If the end-user ceases its business relationship with the LIR, the assigned resources must be returned.

For historical reference Provider Independent (PI) resources are no longer available from RIPE (due to IPv4 exhaustion); PI ("portable") resources are assigned to the end user via a LIR, or directly from the RIPE NCC. They are not managed by the LIR and can be moved between ISPs as the end-user sees fit.

Policy for IPv6 is similar and I've disregarded "IPv4 free market" policies for shortness' sake.

RIPE-599, "IPv4 Address Allocation and Assignment Policies" has the entire policy text for ipv4.