SGP.02 - Remote Provisioning Architecture for Embedded UICC Technical Specification (page 255 of v4.0 specifically) says:
the data format provided by the function caller SHALL NOT depend of the selected OTA protocol capabilities (for example SM-DP can consider there is no limit on data length)
and later
The SM-SR has the responsibility to build the final Command script, depending on eUICC capabilities and selected protocol:
by adding the Command scripting template for definite or indefinite length,
and, if necessary, by segmenting the provided command script into several pieces
and, if necessary, by adding the relevant Script Chaining TLVs.
I understand it that SM-DP can send arbitrary long data parameter to ES3.SendData and SM-SR should send several APDUs in multiple SMSes if data is too large to fit into one. And that is meant by segmenting.
The problem is I can't find a relevant specification that defines how segmenting should be done. And that's the question: where is the segmenting process defined?
I may be wrong but it seems that is not the same as Concatenated Short Messages described in the section 6.3 of ETSI TS 123 048.
It seems that Script Chaining briefly mentioned in ETSI TS 102 226 is somewhat related so pointers to the specification that defines how it works are also very much welcome (TS 102 226 talks about Script Chaining TLVs but not how to use them, at least I'm definitely missing some broader context how it works so any hints are appreciated).
UPDATE:
ES8.EstablishISDPKeySet function requires 3 APDUs to be sent. And they are quite big as they contain keys. From SGP.02-v4.0 table 150 I understand it that they are sent from SM-DP to SM-SR using Expanded Remote Command Format. The script in this format can be rather large as far as I understand (given that SM-DP can assume that there is no limitation on the data length). And it is not clear how SM-SR should segment it or use chaining. I'm just missing the specs where this is described.
The eUICC has a limited internal buffer, i.e. it cannot store a complete profile package of 10kb or more internally. The message must be chunked. If the eUICC supports just e.g. 1 kb then you have to split the APDU commands after at most 3 APDU commands to stay below 1kb. The SGP.02 specification defines to have at least 1024 bytes. A fully featured SM-SR might store some attributes based on the eUICC vendor in the EID to add special handling and patches for certain eUICCs to support a larger buffer size.
Encode each APDUs chunk (1..n APDUs) into the Expanded Remote Command Format (ETSI TS 102 226, sect. 5.2.1) (compact format can only have one response for the last APDU, but if it works, you could save a few bytes)
Encode each Expanded Remote Commands message into a SMS-DELIVER (TS 123 048 and simalliance Interoperability Stepping Stones release 6) This includes the data encryption using the OTA keys (KiC, KID). gsm0348 is a good Java library for this. Pay attention: For each message the OTA counter must be incremented. Select one reference number and keep it for all messages. I guess this is the identifier how the eUICC knows which messages belong together.
If using SMS (I would suggest to use CAT-TP or HTTPs instead -> faster and more reliable) encode it as SMS-PP Download message (TS 131 111). You will using message concatenation here if the payload is more than 140 bytes.
You will receive as response a SendShortMessage (TS 131 111, 6.4.10). Extract the user data again with TS 123 048 and simalliance Interoperability Stepping Stones release 6. You get the SMS Response message. Look into the response packet to get the user data.
Extract the user data as Expanded Remote Response (ETSI TS 102 226)
The eUICC will handle the streamed messages. The concatenated short messages are only used for a message chunk belonging together during the transit.